Specifying Timber Products for Exterior Applications

This short course focuses on the specification of timber products for exterior applications. It covers relevant basic wood properties, different in-service environments as well as legal requirements. It is aimed at architects, engineers and other built environment professionals in South Africa who want to explore the use of wood in structures, including residential buildings. We hope you will start your journey in wood here!

short course

How to complete this course

The course is open access and the content is available to anyone with internet access. This course should be completed in the sequence of the numbered sections. We make use of four components in our course (a) sections with text and illustrations, (b) video clips to explain some of the concepts, (c) a short online quiz after each section to test your acquired knowledge and (d) a test, available to learners who wish to obtain a certificate of completion. Please contact us at info@thewoodapp.com with the short course name to complete this test.

To make the course accessible to most people, we’ve tried to limit the time needed to complete it (an estimated 2.5 hours). This course will give you an overview of selected building materials impact and the environment, and design strategies to incorporate timber in a sustainable way. For in-depth knowledge on some topics you might require input from other sources such as national standards, material suppliers, selected textbooks, articles, etc.
If you have any questions or suggestions, please contact us at


1. Introduction

2. Wood Properties

2.1 Composition and structure
2.2 Wood water relations
2.3 Quiz 1

3. Wood Degradation

3.1 Physicochemical agents
3.2 Biological agents
       Wood destroying insects
       Wood degrading fungi
       Marine borers
3.3 Quiz 2
3.4 Timber use categories, use and hazard classes
       Hazard Classes used in South Africa
       Climate, weather and environments
3.5 Quiz 3

4. Wood Preservation/Protection

4.1 Preservative treatment with biocides in South Africa
4.2 Modified wood
4.3 Surface finishes
4.4 The use of preservative-treated timber in specific areas in SA
4.5 Legal aspects
4.6 Quiz 4

5. Summary


6. References

In the compilation of this module, free use was made of published information such as text, figures, drawings, tables, graphs, etc. As the use of such material is subject to copyright considerations, and the suitability and relevance of this content is in the process of being assessed, the content of this module is only reserved for personal use and the purpose intended. To adhere to copyright regulations, any publication of the module or parts thereof considered, is subject to obtaining the necessary copyright agreement from the publishers by the author. Photographs taken and figures, drawings, tables and graphs generated by the author, are subject to copyright as well.

Whilst all and the utmost care has been taken to ensure the accuracy of the information contained in this module, no warranty can be given regarding the use, suitability, validity, accuracy, completeness or reliability of the information, including any opinion or advice.

Specifying Timber Products for Exterior Applications Video

1. Introduction

  • In the design and planning of structures or constructions, suitable materials are selected and specified to satisfactorily perform under the specific environments or service conditions. To ensure/guarantee the anticipated performance of a material such as wood under the chosen conditions, specific knowledge of and experience with wood as well as the chosen application/in-service environments are required.
  • Designs (and specifications) mainly focus on the functional quality of the materials in the construction, i.e. be it structural and/or aesthetical. Aspects such as how they are installed and how they will be used, are specifically included. For example, ultraviolet radiation protected plastic gutter and downpipes, corrosion resistant metal fasteners and roofing materials, preservative treated timber, strength graded timber roof trusses, surface protective coatings, etc. are specified.
  • Construction, the process of installing the timber product in the structure, and quality of workmanship, is taken care of by the master builder and monitored by the specifier.

To ensure building performance, adequate in-service maintenance is anticipated to be conducted by the owner of the structure, when and where necessary.

During the abovementioned phases, some of the following questions are often asked:

  • What types of built environments exist?
  • Using wood/timber, what can and how is this material degraded when exposed to these environments?
  • How durable is the timber products considered in the design?
  • How can quality timber products be expected to perform, i.e. what is their expected service-life?
  • To be able to address these and related questions, the following topics will be briefly discussed to answer why we must specify the necessary requirements. These are:
    • Wood properties related to performance
    • The built environment surrounding wood.
  • Only when identifying and understanding the types and mechanisms of wood degradation that can possibly take place when wood is exposed to its built environment, suitable protective measures in the design, building and habitation (utilisation) phases can be specified and taken to ensure the expected performance of the materials and buildings.
  • Why/How/What we must specify is briefly discussed under the following topics:
    • Wood degradation
    • Wood protection.
  • To assist architects and engineers designing structures and builders constructing them, the objective of this introductory course is to provide some basic or essential knowledge/information on natural (untreated/unmodified) wood and preservative treated/modified wood as construction material, and why and how they behave when subjected to different built environments.

In any structure built with wood, building science and civil engineering are complementary to this knowledge.




 2. Wood properties

2.1 Composition and structure

During exposure to their “working” environment in constructions, the behaviour of materials such as metals, synthetic polymers, wood or composites such as brick, cement, etc. can all be explained in terms of their physical and chemical properties. Materials such as wood, “manufactured” through a natural process, i.e. biological growth, possess an additional but very influential characteristic, namely its anatomical properties. During the tree’s life where wood is made and used by trees, the physical, chemical and anatomical properties are created and integrated during growth. In a living organism such as a tree, each property as well as the combinations of these three properties, are unique.
We are all familiar with the large variety of tree species, and that different timber types are available for a large range of products manufactured from timber. Generally, timber features appear to be similar. However, every piece of wood cut and used is different in terms of its structure as determined by its growth conditions. Wood is not just wood! Different “structural” entities, which were created during the growth of the tree and which will also affect the conversion of the wood into a construction material, as well as its behaviour in a construction, can be distinguished. These structural entities directly influence the performance of the wood in-service.
At the chemical structural level, carbon, oxygen and hydrogen are the dominant chemical elements used to produce wood, a biomaterial that exhibits a cellular structure. Using water and carbon dioxide as input chemicals, a large variety of chemical compounds is produced to create the cells of the tree. Most of the carbon, oxygen and hydrogen atoms are present in the cell walls as polysaccharides and lignin. A multitude of low molecular mass compounds which have unique properties, and which sometimes also contain small quantities of other elements such as nitrogen, are also present.
Wood consists of various types of cells, each type performing different functions in the tree. The anatomy of the cell walls is determined by the arrangement of the polysaccharides and lignin. When orderly grouped together, e.g. in a growth ring, thick-walled cells are used for mechanical support in the tree while others with large cell cavities (lumen) provide avenues for transport (flow) of sap or enable storage of chemical products. As depicted in Figures 1 and 2, most cells are orientated vertically while some others are arranged horizontally.

Figure 1: Schematic drawing showing positions and orientations of, and connections between, different types of wood cells in a typical softwood such as pine (left) and a typical hardwood such as eucalyptus (right).

Figure 2: Section of a tree trunk/stem showing circular growth (annual) rings pattern and anatomical patterns created on radial, tangential and transverse timber surfaces.

Figure 3: Schematic drawing of sap flow in hardwoods. Arrow size indicates magnitude of flow (parenchyma, vessels and fibres are cell types).

These horizontally and vertically orientated cells are interconnected by openings called pits, in the cell walls to afford sap flow between them (Figure 3). When pits are open, flow is possible in the three main anatomical directions, i.e. longitudinally (parallel to the vertical direction in the tree), radially (from the bark side of the tree to the centre of the stem) and tangentially (parallel to the circular growth rings in the stem). The different cell types (which differ in shape, dimensions and function), their numbers and distribution in the woody material, generate unique anatomical characteristics. These characteristics can be used to identify individual species microscopically, e.g. distinguish between softwoods such as pine, and more anatomically diverse hardwoods such as eucalyptus.
When the more mature trees are ready for harvesting and conversion into timber products, such as poles or sawlogs, four different timber zones related to the growth (age) of the tree become visible.
Figure 4 schematically shows four different zones; two zones that refer to the sapwood and heartwood of a tree (left) and the other two, the juvenile and mature wood zones on the right.
The heartwood/sapwood volume in the tree is shown as two “cones” – the sapwood in the outer side of the stem and the heartwood found inside but not extending towards the top. In the living tree, the sapwood part is responsible for the flow of sap up and down the tree, whereas the heartwood formed earlier, has undergone chemical and anatomical changes that do not allow sap (liquid) flow anymore. In some species, the difference between heartwood and sapwood is not sudden but gradual. This type of wood, called transition wood, obviously has characteristics between sap- and heartwood.

Figure 4: Schematic presentation showing the position of sapwood/heartwood and juvenile/mature wood zones in a tree

New wood growth, which has taken place in the past when the tree was young, and the latest wood formed in the tree while growing upward, is classified as juvenile wood. Juvenile wood is present over the full length of the stem. In terms of wood quality, juvenile wood is not as strong as mature wood and shows different shrinkage and swelling behaviour.
Although sapwood is structurally more or less the same as the heartwood, it usually is much more permeable for liquids (e.g. water) and gases than heartwood. Furthermore, type and quantities of extractives found in sapwood differ from those found in the heartwood. If no colour differences exist, sapwood cannot always be distinguished from heartwood (Figures 6 and 7).
As illustrated in Figure 5, depending on the heartwood/sapwood and juvenile/mature wood patterns, the breakdown of logs will necessarily generate boards with variable volumetric amounts of these four wood types.

Figure 5: Sawlog breakdown example generating marketable board sizes having different quantities of sapwood and juvenile wood

Figure 6: Transverse surface of an airdry, untreated 120 mm diameter Eucalyptus pole. Sapwood/heartwood boundary not clearly visible. Photo: Tim Rypstra

Figure 7: Transverse surfaceof an untreated 120 mm diameter Eucalyptus pole showingsapwood/heartwood areas and sapwood depth after highlighting with colour indicator. Photo: Tim Rypstra

2.2 Wood water relations

In a living tree, all wood cells contain relatively large amounts of water or sap (i.e. water containing dissolved natural compounds). As a direct result of their polymeric chemical composition (i.e. polysaccharides and lignin), structure and arrangement, the cell walls are saturated with water. Some water is bound to the polymers, and the cell walls are, therefore, fully swollen. Variable amounts of water, called free water, are found in the cell lumens called the cell cavities. The amount of water (expressed as its moisture content) present in wood, in the tree or when sawn into boards and dried, can be expressed as a percentage of the oven-dried (at 105±2˚C) mass of the wood using the equation:

Moisture Content (MC) % = 100(Wet Mass – Oven-dried Mass)/Oven-dried Mass

A freshly sawn piece of wood weighing 100 g and which after oven-drying weighs 50 g, therefore, has a “green” MC of 100%. (MC (%) = 100(100 -50)/50 = 100%). E.g. in freshly harvested Douglas fir (Oregon pine), the “green” MC of heartwood MC is around 37% in the log whereas the “green” sapwood is approximately 115%. In trees or poles planted in, i.e. in contact with wet soils, MC’s can often be higher than 100%.

Wood loses water to the atmosphere (environment) when a tree dies or when harvested and the logs are sawn into boards. Under these atmospheric drying conditions, free water is lost first, and removal of bound water will follow. The MC at which no free water is present, i.e. at the point where the cell wall is still fully swollen, is called the fibre saturation point (FSP). Most wood species have FSP’s in the high 20%’s. Lower FSP’s are possible, e.g. teak, (Tectona grandis) a very durable and dimensionally stable wood species, has an FSP of 18%. For general working purposes and calculations, the FSP of wood is taken as 30%.

Figure 8: Mathematical relationship between the moisture content of air (expressed as Relative Humidity in %), air temperature in ˚C, and the equilibrium moisture content, EMC (%), of wood.


25˚C / 80% RH gives EMC = 16%

When wood is exposed to environmental conditions that differ in water content and temperature, MC’s vary accordingly. In the built environment, water can be present as a gas in the atmosphere or in liquid form in contact with wood. Wood subjected to environments (atmospheres) with constant or stable water content and temperatures long enough, will get to an equilibrium moisture content, EMC, with that environment. In air, such equilibrium conditions seldom exist as the temperatures or water contents of these environments change continuously. This affinity of wood for water (its hygroscopicity) is demonstrated as variations in MC levels when changes in environmental conditions occur. Under conditions in an atmosphere where no contact with liquid water such as in rain, dew or irrigation is possible, MC’s will never be above FSP. The relationship between wood EMC (%), water in the atmosphere/air expressed as relative humidity (RH%) and air temperature (dry bulb), is illustrated in Figure 8.

Wood losing water from “green” (freshly harvested) down to FSP does not shrink. Drying below FSP, i.e. loss of bound water attached to the polysaccharides and lignin, leads to shrinkage of the wood. However, due to the anisotropic structure of the wood, which refers to the difference in physical properties in the three main anatomical directions, this shrinkage also demonstrates similar differences in those directions. Between 0 and 30% EMC (FSP), the relationship between shrinkage and MC is essentially linear but differs in magnitude, i.e. they have different shrinkage coefficients. The shrinkage coefficients for normal wood in the three anatomical directions are approximately as follows:
Tangential (8%), Radial (4%) and Longitudinal (0.1%*)
(*Juvenile wood normally has much higher longitudinal shrinkage than 0.1%.)
E.g. when a tangentially sawn board with an MC of 25% is subjected to an environment of 10% MC, (i.e. an increment of 15 percentage points over the full 0-30% range), a % tangential width shrinkage of 4% can be calculated (25-10)/30 x 8 = 4%).

These calculations, performed after conducting appropriate MC measurements, are important for dimension sensitive applications, e.g. when flooring and ceiling boards are installed and the MC’s of the boards and environment are not known.

Figure 9: Tangential and radial shrinkage of Southern pine vs Moisture content (%)

The anisotropic shrinkage and associated deformation when “green” (MC > FSP) square, rectangular and round wood pieces are cut/sawn, and then dried to MC’s below FSP, are shown in Figure 10.

Figure 10: Influence of wood anisotropy on shrinkage and distortion of square, rectangular (flat) and round pieces of timber during drying below FSP. Explained by the fact that tangential shrinkage is about 2x more (8 vs 4%) than in the radial direction.

As expected, when relatively dry timber that has been sawn/cut square, rectangular or round at low MC’s in a factory, is exposed to higher MC’s of an exterior environment, the same anisotropic swelling, up to FSP (c. 30%) occurs as well.

Besides the knowledge of its cellular structure, chemical composition, anisotropic character and relationship between wood and water, and effects associated with it as briefly discussed above, other properties, often showing large variations, are also important or must be borne in mind when wood is considered for use or when its behaviour is explained. These are:

• Density: levels and differences caused by genetic and environmental factors during growth in the tree and between trees. Strongly correlates with strength properties.
• Colour: resulting from the presence of extractives; patterns demonstrated by difference in densities of early- and latewood, presence of sap- and heartwood.
• Odour: resulting from the presence of volatile extractives.
• Porosity: resulting from air cavities such as empty cell lumen and intercellular spaces.
• Capillarity: resulting from the presence of small diameter pathways for liquids.
• Grain: refers to dominant orientation of cells (spiral, interlocked, etc. grain).
• Permeability: refers to the ease with which gases and liquids penetrate into and are released from wood. (Figure 3.)
• Biodegradability: susceptibility to be broken down to simpler compounds by various organisms.
• Combustibility/flammability: measure of how easily wood ignites/bursts into flames; results from the organic composition.
• Glueability/coatability: depending on surface quality, the ability to be glued or coated with surface finishes.
• Workability: refers to working properties such as sawing, peeling, planing, shaping, turning, sanding, drilling, screw-/nailholding, etc.
• Weatherability: ability to withstand exposure to the weather.
• Reactivity: chemical response towards alkali, acids, salts, oxidative agents/circumstances, etc.
• Acoustic: ability to propagate sound waves.
• Electrical: conductivity, dielectric properties.
• Thermal: combustibility, insulator, conductivity, expansion coefficient, specific heat.

2.3 Quiz 1

1. Flow of liquids and gases in solid wood is the easiest in:
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2. During drying, shrinkage of wood is the highest in
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3. In a mature tree, sapwood is found
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4. The moisture content of a piece of airdry wood with a volume of 0.5 m3, weighing 300 kg and having an ovendry mass of 200 kg, is
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3. Wood degradation

Wood degradation (a process) is a multidisciplinary field that requires knowledge of the chemistry, anatomy and physics of wood, as well as of applied biology, chemistry, physics, mechanics, environmental science, etc. Wood degradation or failure can be defined as the inability of the timber product to satisfactorily perform its function in a construction. e.g. structural, (members of a roof truss, foundation poles, steps, etc.) and/or aesthetic (weathered exterior wall cladding, fascia boards, etc.). (Because processed wood is used in constructions, this course does not focus on degradation wood in the living tree.)

Two classes of agents can degrade wood:

3.1 Physicochemical agents

• Heat: Decomposition and the associated loss of mass of wood start when heated above approximately 100 ˚C. Between 100 ˚C and 200 ˚C, the decomposition is slow; more water is driven off along with CO2 and some CO. The wood will gradually degrade or pyrolise. More rapid pyrolysis occurs above 200 ˚C and it will accelerate from 260 ˚C to 350 ˚C. Flammable gases evolve. With enough air (oxygen) present, these gases can ignite either from an ignition source such as a flame, or they will self-ignite if the temperature is high enough. At about 270 ˚C the rate of heat produced will be greater than the heat required to generate the gas, and the fire can support itself. At 500 ˚C, glowing charcoal is oxidised to leave ash.

Burning will continue if the wood can be maintained at a sufficiently high temperature.Dimensions (e.g. particle size) and shape, orientation and MC influence the burning process. Timber > 15 mm in thickness cannot continue to burn unless extra heat is supplied as an insulating charcoal layer is formed at the surface.

The effect of heat other than leading to a fire is often not considered. However, the level of and change in temperatures to which wood is subjected during exposure to outdoor or even indoor conditions, combined with the more damaging associated changes in MC, observed as intermittent swelling and shrinkage, e.g. during weathering, can be substantial.

• Light: can cause discolouration of wood (lightening or darkening) as well as photodegradation of wood components, e.g. lignin. The latter resulting in superficial fibre separation and the slow erosion of the wood surface.

• Chemicals: These include alkalis and acids, leaching water from asbestos/cement materials, salt laden air from the sea, corrosive deposits from corrugated iron roofs, nails, screws, etc., and can attack and chemically degrade wood. Chemical reaction rates are increased when temperatures increase, i.e. the rate roughly doubles for each increase of 15˚C.

• Mechanical forces: unbalanced compression, tensile, bending forces, creep, etc. due to improper design and use.

• Weathering: a combination of the four above. Sunlight (electromagnetic radiation containing ultraviolet light (UV), visible light and infrared light (IR)), wind, liquid water (rain, dew, fog) and water in the gas phase (humidity), all play a part. Water is acting here as a physical degradation agent. While repeated swelling and shrinkage occur (i.e. acting as mechanical forces), surfaces become rough, fibres are removed from the degradation of the surface and due to associated swelling and shrinkage, deeper checks and end splits develop.

The opening up of the wood material associated with changes in MC, provides conditions for spores of wood destroying fungi to penetrate deep(er) into the wood, making fungal attack possible. Species, dimensions (shape and size), orientation and exposure direction in the building, climate, geographical location, etc. influence the intensity and rate of weathering.

3.2 Biological agents

Wood destroying insects

Borer beetles

The relative amount of damage caused by wood boring beetles differ from place to place. The life cycle of beetles, the most highly developed insects, is shown in Figure 11. The flying adult insect lays eggs in or on the wood, they then hatch and change into larvae. Provided that favourable living conditions exist, including MC, the stage where the larvae tunnel into the wood, leaving wood powder/frass behind, causes most of the structural damage to the timber product. After changing to the pupa stage, the flying insect then emerges from the timber. The flight or emergence holes, and frass pushed out of the holes or lodged in the tunnels, can be indicative of the species. The life cycle period can vary. The attack can be wood type and geographically specific. Larvae tunnels created in the timber can significantly reduce its strength.

Figure 11: Typical wood boring insect life cycle

The most important wood boring beetles in SA are listed below, including some of their characteristics useful for identification, such as size and shape of flight holes, frass texture, wood species attacked and geographical location of possible infestation:

Ambrosia beetle: attacks green or freshly felled wood, dark discoloration on inside of tunnels visible; no frass present, circular flight holes, mostly found in hardwoods.
Powder post beetle (Lyctus brunneus): attacks the sapwood of seasoned hardwoods such as eucalyptus and oak species, small circular flight holes and fine, powdery frass. Found everywhere in SA.

Figure 12: Lyctus brunneus attacks in dry timber. (Note circular flight holes)

  • Furniture beetle (Anobium punctatum): attacks seasoned and moist soft- and hardwoods; modern and antique furniture, exterior and interior joinery, joists, beams, flooring, doors and door frames. Circular flight holes and powdery frass. Found everywhere in SA.
  • Italian beetle or European houseborer (Hylotrupes bajulus): attacks all seasoned SA grown pine and other softwoods in the coastal regions, especially in the Western Cape Province; oval flight holes, coarse frass.

Figure 13: Dry softwood attacked by Hylotrupes bajulus

Wood destroying termites

Termites are social insects and live in colonies in total darkness in their nests.

• Dry wood termites (Cryptotermes brevis): these flying insects attack all seasoned wood species by air in coastal regions stretching from Gqeberha (Port Elizabeth) to Durban. Gritty frass. Irregular shaped flight holes.

Figure 14: Cryptotermes brevis attack in exterior door

  • Subterranean termites: attack all hardwoods and softwoods above and in the ground. Wood is consumed by one specialised caste, the workers, and digested in fungal gardens. These termites live similarly to ants in colonies in the ground. The mounds of each species have a characteristic shape. They enter the wood from the ground or through shelter tubes they construct. They can attack sound wood but prefer wood that has already been attacked by fungi. Attack is not easily visible on the surface of infested wood.

Distribution: can be found anywhere north from a horizontal line roughly drawn through Paarl.


Figure 15: Attack of floors by subterranean termites showing tubes (left) and subfloor mound (right)

Figure 16: Subterranean termite and fungal attack of poles

Wood degrading fungi

Generally, plants require water, food (nutrients), heat and oxygen to live. Fungi are plants that do not have chlorophyll to manufacture their own food using light. Depending on favourable conditions, they can live on dead or living matter. As seen in the life cycle depicted in Figure 17, they reproduce by forming numerous airborne spores that can, under favourable circumstances, settle on surfaces suitable for germination. Optimum fungal growth on wood occurs at temperatures between 20-30˚C, enough oxygen, a wood MC > 20%, the absence of light, and chemical compounds in wood that are not toxic. Attack can take place anywhere (in the world) when favourable growth conditions are created and maintained.

Figure 17: Life cycle of fungi

Wood destroying fungi: they secrete enzymes that can specifically dissolve the structural components of wood making up the cell walls, i.e. polysaccharides and lignin. These fungi belong to the Basiomycetes and the Ascomycetes groups and attack is classified based on two criteria, MC or colour of the wood that is attacked.

The effect on wood is the darkening of the wood. The development leads to cuboidal cracks, decreased density, brittleness, loss in strength, and a musty smell.

Attack can be identified by colour as:
• Brown rot: the wood residue is brown in colour as most of the polysaccharides have been dissolved enzymatically and a brown, lignin rich residue is left behind.
• White rot: a fibrous, white, cellulose rich residue remains after most of the lignin and some celluloses have been removed.

Figure 18: Typical brown rot attack. Cuboidal cracking along the grain

Figure 19: Typical white rot attack. Light coloured, fibrous structure.

Identified by MC:

  • Dry rot: Found in wood that is kept at optimum MC levels in the range from 30 – 40%. Most of the time, this kind of rot is found under interior circumstances, i.e. inside buildings, when water leakage and/or insufficient ventilation/drying conditions exist.

Figure 20: Dry rot in ceiling due to leaking roof

Figure 21: Surface texture and colour of dry rotted timber

  • Wet rot is present when MC levels range between 50-60%. Parts of exterior structures such as bottoms of door and window frames (coated or not), parts of wooden steps, balustrades, deck boards in contact with their substructures, etc. can be attacked.

Figure 22: Wet rotted decking timber

  • Soft rot: Under very wet conditions such as in cooling towers, vineyard poles planted in flood or dripline irrigated areas, the peripheral surfaces of the timber product are attacked by these soft rot fungi. They belong to the Ascomycetes

Figure 23: Soft rotted timber

Non-wood destroying fungi

  • Sapstaining fungi can attack freshly felled timber, or dying trees attacked by forest insects or damaged by fire. They also live on timber in-service, kept wet for longer periods such as underneath varnishes. They live on the cell lumen contents and are, therefore, not regarded as having an impact on the strength properties. Acceptance of the dark or blueish coloured timber is subjective and when used for aesthetic purposes (e.g. ceilings), it requires agreement between supplier and customer prior to installation. Due to the deep occurrence, these fungi cannot be removed chemically, e.g. by using bleaching agents.

    Figure 24: Sapstained timber boards

    Figure 25: Blue stain in-service underneath clear coating (left) and painted surface (right)

    • Specific surface mould fungi live on the surfaces of timber products (and other materials) but also do not affect timber structurally. Various colours other than brown or black are possible. The timber substrate often provides the moist conditions favourable for this type of fungus. It typically develops when timber is close stacked immediately after treatment with waterborne formulations, or when dry, planed/sawn timber is exposed to environmental conditions associated with water precipitation, e.g. rain, dew or wood surfaces that are wet for long times. Mould can be removed chemically using a bleach solution.

    Figure 26: Surface mould on pine decking

    • Bacteria
      Not much information is available on bacterial attack in buildings.Timber objects submerged undersea have shown pockets of attack.

      Marine borers
      Harbour posts, pilings and wooden boats in contact with seawater or brackish water, can be attacked by three types of small marine animals called marine borers. They are widely distributed over the world. The following species are responsible for attack:

      Shipworms (species of Teredo and Bankia).
      Pholads (principally Martesia), which both are related molluscs that are related to clams and oysters.
      Crustaceans, principally species of Limnoria, Chelura and Sphaeroma, are related to crayfish.

      Larvae of shipworms attach to the wood surface, making a small entry hole and then making galleries lined with shell-like material, along the grain until the wood is completely honeycombed. They typically concentrate near the mud line of a post.

      Pholad damage is similar but borings are shorter.

      Limnoria attack on wood pilings is less catastrophic than that of shipworm or pholads because it is more easily detected and occurs in the surface layer. Principal attack is on wood exposed between high and low tides. Wave movement erodes damaged material away giving the post an hourglass shape.

    3.3 Quiz 2

    1. Wood is fully burnt to ash when exposed to heat at
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    2. Weathering affects
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    3. Attack of the heartwood of a SA pine pole by wood destroying fungi is possible when the following wood moisture content conditions exist:
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    4. The European houseborer (Hylotrupes bajulus) can attack a cross laminated eucalyptus timber panel in
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    3.4 Timber use categories, use and hazard classes

    To somehow predict/ensure/guarantee/specify that wood in a chosen structure or application will perform as expected, i.e. sufficiently protected in its environment to last long enough or that the product is not prematurely degraded by Moisture, Organisms, mechanical Stress and Heat (MOSH), the following objectives, and possible actions, should be considered:

    • That the structure does not collapse due to unbalanced mechanical forces (Stresses) – ensure correct structural design and proper selection and installation of suitable (strength graded) materials.
    • That the strength of wood members/assemblies/structures is not reduced by:
    • Biological degradation agents (Organisms)– use naturally durable wood or preservative treated/modified wood so that it is more bio-resistant than untreated, non-durable wood.
    • Heat – use fire retardants and effective fire control measures.
    • Exercise Moisture content control – architectural design, use modified wood and/or apply surface finishes, and perform regular (Wood preservatives/biocides, excluding creosote, can normally not control MCs on their own).

    Based on knowledge and experience gained over years, specific influencing factors were identified in the compilation of classification systems that describe different environments where timber is used. These factors are:

    • Whether timber is used
    • Under interior (inside/indoors) or exterior(outside/outdoors) conditions and will be in contact with air (the atmosphere), the ground or water or combinations
    • A further important consideration in a classification is how long and how frequent the wood is exposed to unfavourable use (in-service) conditions, i.e. a description of the time — Is exposure short, continuous or does it occur periodically (seasonally), etc.? E.g. it is logical that a too short exposure to favourable conditions that favours fungal growth, does not necessarily manifest as fungal damage.

    Many possible combinations of air/ground/water/time of the working environments of the wood can exist, therefore, further subclassification is logical and necessary. E.g., during ground contact, soil types and their characteristics such as water holding capacity and atmospheres (weather and climatic conditions) surrounding the wood do vary, and have a profound effect on the type, rate and extent of degradation. Over the years, the type of water, i.e. fresh water or sea water, that timber is exposed to, has worldwide been recognised as an important subclassification factor.

    For the purposes of determining what level of protection is required, e.g. preservative treatment and/or other measures, should be taken to ensure an acceptable service life of wood in its chosen environment, two approaches to classification systems are used worldwide:

    • a numbered Use Category or Use Class (UC) system which essentially specifies various in-service conditions starting at relatively mild interior/indoor conditions (e.g. UC1) and ascending in number to the much more severe/degrading marine (exterior, salt water) conditions (UC5);
    • a numbered Hazard Class (H) system, which works on the same basis, the numbers used not necessarily being the same as those used in the UC system. E.g. H2 (interior) and H6 (marine).

    In South Africa, and as stipulated in SANS 10005: Preservative treatment of timber, the Hazard Class classification system is used. The Hazard Classes and where (exposure conditions (Use Categories)) and how the wood is used/recognised in SANS 10005, are listed in Table 1. Influencing criteria, i.e. water, ground and time are highlighted.

    Table 1: Timber Hazard Classes recognised in SANS 10005


    • Classification criteria:

    Water (type: sea/estuarine/fresh; wet; weathering; leaching)


    Time: (constantly/periodically; not (never))

    • Classification focus: On biological agents and their environmental requirements. It does not include heat.
    • Some products can be exposed to one or more hazard class simultaneously, e.g. H2/H3 beams or H3/H4, H3/H5 or H3/H6 poles.

     In practice, and from the wood/water relation background description given above, some further subclasses of the Use Category, Use Class or Hazard Class system become necessary. E.g., the exterior SA above ground (air/atmosphere) exposure class (H3) should preferably distinguish between conditions where protection from rain/precipitation/direct sunlight is present or when fully exposed, open-air exposure takes place. Compared to classification systems used in North America, the European Union and Australia (including New Zealand), such subclassification is not yet present in the SA Hazard Class system. The UC systems used in North America and the European Union, and the Hazard Class system used in Australia can be regarded as more detailed and descriptive. A brief discussion of the USA, European and Australian systems that provide more detailed information for design and specification purposes is provided in the file, USA_EU_Aus_Use_and_Hazards_Classification.pdf, attached.

    Hazard Classes used in South Africa
    Table 1 (shown earlier), based on the hazard classification listed in SANS 10005, can also be expanded to include the agents that are operative under these classes (Table 2). While attack by fungi is possible in all five hazard classes and anywhere in SA (in the world also), the threat of insect attack in classes H2– H5 is governed by wood species, e.g. softwoods or hardwoods, and geographical location. E.g., drywood termites can be found in both H2 and H3 environments but in the coastal regions from Gqeberha (Port Elizabeth) to Durban only. Similarly, the powder post beetle, Lyctus brunneus, can attack the starch rich sapwood of hardwoods, e.g. eucalyptus products (pole or sawn) located under H2 and H3 atmospheric conditions anywhere in SA where conditions are favourable, if not preservative treated.

    Table 2: Hazard classes and potential degradation agents

    Climate, weather and environments

    The more detailed USA, European and Australian classification systems discussed in the PDF given earlier, and the brief reference to the Hazard system used in SA, indicate that MC, a specific wood parameter, and time of exposure, are both difficult to quantify, classify and, therefore, to specify. One specific MC value and the time-related property though, are very important. From a fungal attack point of view, it is important that it should not stay above 20% for “a long time” as chemical protection with biocides would then become necessary.

    The foregoing attempts to correlate climatological information of the use environment to the resistance of wood against attack by wood destroying fungi when used above ground (H3 – conditions), are not that successful in estimating specific MC’s, and variations for the areas where wood is used. The results of long term, in-service MC measurements of wood subjected to different but typical conditions are required.

    One set of data of interest to sawmillers and timber processors are the typical EMC’s that prevail when timber is exposed to exterior, under roof conditions, i.e. not exposed to wetting from different forms of precipitation and drying caused by direct radiation from the sun (Figure 27 and Table 3). To control dimensional changes associated with changes in MC’s of timber products, the sawmiller should dry the timber to ± the same MC’s at which the timber is going to be processed further and used. This specific MC is also an important quality control step for timber treatment with water-borne biocides/preservatives that causes the wood MC to increase far above 30%, i.e. the FSP. As expected, the coastal regions that include cities and towns such as Cape Town, George, and Gqeberha (Port Elizabeth), i.e. especially the more southern parts, and Durban to a lesser extent, show the highest annual average MC’s. The lower MC values observed further inland can best be explained by comparing the EMC with a climatic map (Figure 28). Figures 27 and 28 show a similarity in patterns. Where Figure 28 focuses on a description of climates in terms of temperatures and precipitation for energy purposes, Figure 27 describes the SA climate in wood related terms, i.e. the average annual EMC, and its variation (Table 3), of wood exposed to under roof exterior conditions in different geographical areas. These under roof exterior conditions refer to environment/atmospheric conditions where there is no direct exposure to the sun’s radiation, and to any form of free water, i.e. exposure to moisture in gas form/humidity alone.

    Figure 27: Approximate EMC’s for wood in various areas of South Africa (Van Vuuren et al 1978).

    Table 8: Average and seasonal variation of EMC in different localities

    Figure 28: Climatic zone map (SANS 204-2, 2008). Energy efficiency in buildings.

    Climatic conditions determine the presence of specific insects in certain geographical areas. What is noticeable is that Hylotrupes bajulus is mainly found in the temperate coastal areas of the Western and Southern Cape provinces (blue) and that Cryptotermes brevis is found in the sub-tropical coastal areas surrounding Durban (green). A similar map of rainfall patterns (when, how much, how long) could also be quite useful to explain the MC patterns at a location or geographical site.

    The average annual MC’s shown in Figure 27 do not demonstrate the monthly EMC’s and the MC variation pattern throughout the year. This variation, for the two locations in SA, i.e. Cape Town and Pretoria, calculated from monthly weather data were reported by Simpson (1998) and is illustrated in Figure 29. The patterns of summer and winter EMC conditions (Figure 30) are quite noticeable; i.e. lower MC’s in the PTA winter months and higher ones in CT during the same period.

    Figure 29: Annual variation in average monthly EMCs of wood in Cape Town and Pretoria

     Figure 30: Rainfall patterns in Cape Town and Pretoria (Weather-atlas.com)

    Both EMC (Figure 29) and rainfall patterns (Figure 30) still do not explain the delivery of water on a daily (weather) basis. From experience we know that a consistent, extended, low quantity rain period can create much higher MC’s than when the same amount of precipitation is delivered, e.g. during a thunderstorm. At face value and working with average monthly MC’s only, the levels and ranges (Max-Min) of the two locations, CT and PTA, do not differ dramatically. The highest monthly MC of 15.7% for CT in July is still relatively low enough beneath the 20% mark. Clearly, only taking monthly averages of events into account do not reflect what happens on a daily or diurnal basis (weatherly) at the air/wood contact interface.

     Although climatic information can provide some indication of the potential of fungal attack to timber under H3 conditions, the obvious question asked is: “What MC levels and rate of changes occur when wood is exposed to changes associated with weather (daily) conditions?” The position and orientation of the wood surface in contact with the atmosphere (i.e. its environment) as well as the inherent wood properties including dimensions, will play an important role in this.

    Two scenarios that refer to MC variation on a daily basis are presented in Figures 31 and 32. Two thin slivers of Limba wood (representing the surfaces of a wooden object) were exposed to the weather at Stellenbosch, South Africa, on a clear summer day, 21 December, the longest sun hour day of the year, and a rainy, cloudy day, 20 June, the shortest sun hour day (Rypstra 1995). The one sliver was not sheltered, (n), i.e. fully exposed to the elements, facing true north, 45˚ to the horizontal. The other sliver was kept in the open air, sheltered (s) under cover, allowing free air circulation. MC’s and temperatures of the exposed and sheltered slivers, MCn, MC’s, Tn and Ts respectively, and solar radiation were recorded at 15 min intervals for the 24h period. The maximum, minimum and averages of the four parameters were calculated.

    Under these specific conditions, and especially in the clear summer day case, the diurnal patterns are not reflected in the MC and temperature averages calculated. MCn varied between 6 and 28% MC, i.e. it was wetted to 28% and dried to 6%. MCn on a rainy, cloudy winter’s day remained between 20 and 56% most of the time (Figure 32), while sheltered exposure generated MC’s that varied between 17 and 27%.

    Very “responsive” wood surfaces, be it as a result of their position, orientation in a building or having inherent properties that accommodate fast uptake of any type of precipitation AND rapid loss of water when favourable drying conditions set in, would show severe weathering effects. However, the length of time that wood underneath the surfaces remains at MC’s > 20%, is the parameter that should be used to decide whether high pressure application of fungicides (chemical preservation) is necessary. In this concise explanation of this exposure phenomenon and mechanism, the role of surface finishes, and modification of wood that address the hygroscopic nature of the wood and/or reduce the permeability, is particularly important.

    Figure 31: Variation in Insolation (SolRad), Moisture Content (MC) and Temperature (T) of wood, fully exposed/not sheltered (n) north facing at 45˚ to the horizontal and under shelter (s) during a clear summer’s day (21 Dec 1985)

    Figure 32: Variation in Insolation (SolRad), Moisture Content (MC) and Temperature (T) of wood, fully exposed/not sheltered (n) north facing at 45˚ to the horizontal and under shelter (s) during a rainy, cloudy winter’s day (20 Jun 1986)

    A full year’s measurement of MC’s taken every 15 mins and results presented as weekly maximum, minimum and average MC’s, is shown in Figure 33. Averages show a gradual tendency upwards, towards 20%, from week 29 onwards, associated with autumn and winter conditions. Lower than 15% MCn’s were experienced in the spring/summer seasons. The weekly ranges are wide, minimum MCs c. 5% and maxima in the 25-35% range. The >30% MC maxima indicates that the exterior sheltered surfaces were also affected by foggy conditions, i.e. contact with fog droplets in the air present under the roof shelter.

    Figure 33: Weekly average MC of wood kept under roof; from 10 Aug 1988 to 9 Aug 1989


    3.5 Quiz 3

    1. The harshest environment for wood is
    Field is required!
    Field is required!

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    2. The following statement is not true:
    Field is required!
    Field is required!

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    3. The following parameters are used in the compilation of hazard classes:
    Field is required!
    Field is required!

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    4. The following statement is not true
    Field is required!
    Field is required!

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    Try again.

    4. Wood Preservation/Protection

    Wood protection, by definition, covers ALL processes or measures taken that enhance the lifespan (i.e. the function) of wood, be it structural, aesthetical or both. Where many efforts to protect wood are still focused on preservation, i.e. the application of preservative chemicals (biocides), newer developments in the developed countries now address wood protection in a wider, more sustainable context. “What will happen to large quantities of timber preserved with toxic chemicals in buildings or constructions after the building has been demolished?” is the question that is asked today. To address this global concern, more ecofriendly preservative formulations and protection processes have been developed over the last few decades.
    In SA, and elsewhere in the world, a large variety of product specifications with an emphasis on performance, and manufacturing codes and standard test methods, have been established. E.g. ASTM, DIN, BS, SABS and industry organisations such as AWPA, SAWPA, SAPMA, PMA, etc. develop, maintain and enforce standards. Standards and supporting test methods that focus on the preservative treatment and preservative treated timber are well developed whereas similar standards or codes related to modified wood, a new approach in wood protection, are still in the process of being developed.

    South African Standards relevant to the protection of wood are no exception. They also mainly focus on the preservative treatment with biocides. Most of the information presented below will therefore focus on South African standards SANS 10005, SANS 1288 and associated product standards. However, some background information on the natural durability of wood is warranted for situations where so called “green” technology, using natural timber products, is considered.

    Natural durability
    As discussed previously, species and parts of trees differ in their resistance towards wood destroying agents. Sapwood, for example, is normally regarded as non-resistant to fungal and insect attack due to the presence of nutrients. Natural resistance is determined by anatomical and physical properties, but especially chemical properties. Natural durability of wood is described as its resistance to fungal, insect and marine borer attack.

    Fungal (decay) resistance
    No timber (over time) is immune to decay, and especially sapwood is always perishable. The heartwood from some species such as Tectona grandis (teak) and Eucalyptus fergusonii (ironbark) can last for decades, even in ground contact. The type and quantity of extractives present in the heartwood determine decay resistance. In the past, the heartwood durability of timber was classified as follows:

    Table 4: Durability classification based on expected life-time.

    Insect and marine borer resistance

    In this case it is more difficult to classify timbers than the resistance to decay (fungi) as shown in Table 4. The reason is that if the timber is susceptible to attack, the rate at which the timber is destroyed depends mainly on SIZE and AGE of attacking population. These two characteristics can vary from place to place.

    Wood-boring insects

    Sapwood is susceptible to attack by wood-boring insects as it contains no toxic, repelling, unpalatable extractives. It is also rich in food, e.g. starch, which supports Lyctus attack. Heartwood varies in susceptibility to different insects (Afrormosia and Sapele are immune to A. punctatum; but are readily damaged by termites).

    Marine borers

    Sapwood is never and heartwood not completely immune. Basralocus, for example, contains high amounts of silica and Jarrah contains toxic repellents.

     Today, EN 350:2016: Durability of wood and wood-based products – Testing and classification of the durability to biological agents of wood and wood-based materials, is the European standard that gives guidance on methods for determining and classifying the durability of untreated and preservative treated wood and wood-based materials (such as modified wood) against biological wood-destroying agents.

    The wood-destroying agents considered in the standard are:

    • wood-decay fungi (Basidiomycetes and Ascomycetes)
    • beetles capable of attacking dry wood
    • termites
    • marine organisms capable of attacking wood in service.

    Data on the biological durability of selected wood species considered of economic importance in European countries, are presented in EN 350. This provides information on their geographical origin, density, sapwood width and treatability.

    The following durability classes (DC) are recognised in EN 350:

    Table 5: Durability Classes (DC) of solid wood (heart and sapwood) according to EN 350

    4.1 Preservative treatment with biocides in South Africa

    In the preservative treatment of timber with biocides, i.e. protection against biological agents, some general properties are required during application, use and disposal. They should be:

    • Safe to use (i.e. of low toxicity, fire risk and corrosivity to metals)
    • Effective (should penetrate deeply, have uniform distribution and consistently prevent attack)
    • Last for a “long time” (should not decompose or get chemically modified, evaporate or leach out)
    • Commercially competitive (should be cost effective in terms of the preservative, treatment and maintenance costs)

    SANS 10005 – The Preservative Treatment of Timber and SANS 1288 – Preservative-treated Timber

    Everything that deals with the preservative treatment of timber in SA (against agents of biological attack) is specified in these two standards. Both updated standards were published in 2021.

    They do not cover the treatment of timber against fire or the modification of wood. The latter, viz. wood modification, is a treatment that focusses on protection against biological agents as well as dimensional stabilisation, i.e. methods that control moisture induced movement of timber.

    The scope of SANS 10005 covers the following aspects:

    • Classification of timber preservatives
    • Hazard conditions for timber
    • Solvents used for timber preservatives
    • Preparation of timber for treatment
    • Treatment processes
    • Use of preservative treated timber in SA
    • Handling and safety recommendations

    SANS 1288 specifies acceptable preservative treatment requirements for preservative treated timber and timber products when exposed to a range of hazard conditions, as described in SANS 10005.

    Several related documents (standards) that refer to the preservatives that may be used, supporting test methods and specific treated products, e.g. hardwood poles used in wet soils, are referenced in both SANS 10005 and SANS 1288.

    In this overview, not all the detailed information but only some aspects of the standards, are discussed. It is essential that original copies of these two and other relevant standards should be purchased and studied.

    At this stage, the important roles played by the South African Wood Preservers Association (SAWPA) (https://www.sawpa.co.za/, should be mentioned. SAWPA promotes timber treatment and treated timber products, and provides an excellent range of articles, brochures, newsletters, technical and industry information, etc. related to their mission on their website.

    As stated in SANS 10005, the following nine SANS approved timber preservatives are currently accepted for use in SA and are also registered with the Department of Agriculture, Forestry and Fisheries as agricultural remedies used for wood preservation under Act 36 of 1947):


    • Three Class C (Coal tar creosote and with a coal tar creosote basis) types:
      • Type 2: Creosote,
      • Type 3: Creosote/Tar solution
      • Type 4: Mixtures of creosote and waxy oil
    • Four Class W (solutions of compounds in water) types:
      • ACQ: Mixtures of alkaline-copper-quaternary compounds
      • CCA: Mixtures of copper-chromium-arsenic compounds
      • B: Borate
      • CuAz: Mixtures of copper azole compounds
    • Two Class O (solutions of compounds in light organic solvents (LOSP’s) types:
      • TBTNP: Tributyltin naphthenate-permethrin
      • ZP: Azole-permethrin

    In addition to the tested anti-fungicidal and anti-insecticidal properties of these nine timber preservatives, several other characteristics related to their application to use, disposal of the timber product, etc., can influence the choice to specify them for a range of applications and environments (hazard classes). These characteristics, mentioned in SANS 10005, are listed in Table 6. (Abbreviations of water- or solvent borne preservative types are indicated with a W or O prefix respectively.)

    Although specifiers and end users are not involved in the timber treatment process, several operational actions that can have an effect on the performance of the treated timber products in their application (environment), must be followed by registered timber treatment operators producing these products in accordance with SANS 10005 and SANS 1288 requirements. Pre-treatment operations such as the preparation, sorting and seasoning of timber, contribute to the quality of treatment and treated product. Post-treatment actions such as handling, storage and installation can also influence treated timber performance in the construction.

    Some examples demonstrating the level of detail associated with these processes before treatment are:


    • To achieve the recommended penetration, adequate sapwood depth is required.
    • The glue of composite timber products such as in laminate beams, must be fully cured.
    • Timber treated with Class W preservatives shall only be glued with Class 3 glues (SANS 10183-2).


    • Usually, no mixing of hardwoods and softwoods in a charge. Limited mixing is allowed but only in accordance with the parameters specified in SANS 10005.
    • Usually ensure that form and dimensions, especially thickness, are similar.
    • Only treat timber that is free from defects, such as
      • decay
      • insect attack
      • resinous patches
      • gum veins (other than veins that occur below the required minimum depth of penetration) and where
      • splitting is less than specified by the purchaser.


    • Timber should be seasoned to the following average MC’s:
      • Treated by pressure process, and unless a different MC is specified in an applicable national product standard, e.g. SANS 1783 parts 2 and 4, SANS 457-2, SANS 753/4:
        • Class O: 170g/kg (17%)
        • Class W and Class C: 300g/kg (30%)
      • Treated by diffusion:
        • Freshly felled or wet off saw.
      • Treated by pressure-diffusion process (Type WB-preservative):
        • No specific requirement, as long as retention and penetration requirements are met.
      • Insect attack on sapwood (e.g. hardwoods) should be controlled with a suitable timber protectant.
      • During the WB diffusion process, apply a mouldicide or mix it with the WB, to protect the timber against superficial mould during close stacking.

    Surface preparation

    Only treat timber free from:

      • Precipitated water
      • Dirt
      • Contamination
      • Fungal attack
      • Insect attack
      • Outer and inner bark


    • Perform all shaping and drilling before treatment, e.g. sharpening of one end of poles such as vineyard poles intended for impalement, is not recommended as it could lead to premature failure due to insufficient sapwood at these ends.


    • This can improve penetration and reduce surface checks. Method and pattern must be agreed upon by purchaser.

    The success of preservative treatment with biocides is very much determined by the ease of penetration of the biocides (in gas or liquid state) into the wood material and how much is retained, i.e. the retention. As described earlier, the permeability of the wood, which is related to the anatomical structure (e.g. species) and sapwood content, are important physical properties to achieve adequate penetration (Figure 34).

    Table 6: Characteristics of SANS 10005 acceptable timber preservatives

    Figure 34: CCA penetration in Pinus sylvestris and Picea abies
    (stained, sapwood/heartwood boundaries are indicated with arrows)

    • In SANS 10005, the permeability or ease of penetration or treatability of the sapwood and heartwood of species available/used in South Africa, is broadly classified as amenable or moderately unamenable to impregnation.

    Species that are amenable, which means the sapwood can be easily impregnated, the heartwood only sometimes are:

    SOFTWOODS                                      HARDWOODS

    Pinus canariensis                              Eucalyptus fastigata

    1. caribaea E. fraxinoides
    2. elliottii                 E. grandis
    3. patula
    4. pinaster
    5. roxburghii
    6. taeda


    Species that are moderately amenable, i.e. the sapwood can be fairly, easily impregnated, the heartwood rarely are:

    SOFTWOOD                                        HARDWOODS

    Pinus radiata                                      Entandrophragma  cylindricum (Sapele)

                                                                                    Eucalyptus citriodora

    1.                                 cladocalyx
    2.                                 cloeziana
    3.                                 diversicolor
    4.                                 maculata
    5.                                 maidenii
    6.                                 microcorys
    7.                                 paniculata
    8.                                 pilularis

                                    \                                              E. resinifera

    1.                                 saligna

                                                                                    Shorea spp. (Philippine mahogany)



    • The primary industrial timber treatment processes allowed under SANS 10005, are briefly described below:
    • Diffusion (Suitable for Class WB preservatives. Used on unseasoned timber. Timber is close-stacked and covered after dip treatment. Temperature increases speed-up the diffusion rate during diffusion storage.)
    • Hot or cold open tank process (Suitable for Class C preservatives)
    • Low-pressure process (Suitable for Class O preservatives)
    • Vacuum impregnation (double vacuum) process (Suitable for Class O preservatives)
    • High-pressure processes (three types), known as the
      • (Bethell (full-cell) process
      • Rueping (empty-cell) process
      • Lowry (empty-cell) process
    • The high-pressure processes, which are the most effective ways of applying a preservative to timber, consist of the following basic steps:
    • timber is placed in a treatment vessel and closed
    • the preservative is forced into the timber under hydraulic pressure
    • the pressure is maintained until the required absorption is attained
    • The three processes differ in that the pressure phase may be preceded by

    a period under vacuum (as in the case of the Bethell (full-cell) process) or

    a period under air pressure (as in the case of the Rueping (empty-cell) process) or

    no preliminary vacuum or pressure is applied (as in the case of the Lowry (empty-cell) process).

    • After the pressure cycle, the vessel is drained; a final vacuum is then applied to adjust the preservative absorption and to “dry” the surface of the treated timber. In the “empty-cell“ process, the surplus preservative is removed by the expansion of air compressed in the cell spaces while sufficient preservative is left in or on the cell walls to give adequate protection. In the “full-cell“ process, the cell spaces are also filled or partially filled with preservative.
    • Bleeding of Class C preservatives aftertreatment is less likely when the empty-cell rather than full-cell process has been used.
    • The empty-cell process allows the achievement of deep penetration and the required net retention. There is usually less variation in net retention in individual pieces of timber in a charge.
    • Pressure processes in general can force preservatives deep into timber (deep penetration) and achieve high retentions and are widely used to apply
    • creosote
    • type WCCA
    • type WCuAz
    • type WACQ
    • type WB preservatives.
    • Detailed descriptions of three processes appear in SANS 10005. The Bethell (full-cell) process can be seen in Figures 35 and 36. A video showing the process can also be seen at:


    Figure 35: Timber treatment cylinder

    Figure 36: Typical steps during full-cell (Bethell) timber treatment

    Handling and safety of preservative treated timber (see also SANS 10255)

    Directly after treatment, several other procedures are followed before dispatched to storage or to purchasers. These include marking the products to indicate the hazard class, name of the manufacturer, year of manufacturing, etc. Sometimes, delivery to purchasers or the building site or even installation occur soon after treatment. Unfavourable or some unforeseen conditions can have an impact on the quality of treated products. Some procedures that require attention are:

    • During treatment, timber treated with Class W preservatives becomes wet. Being heavier, uncareful handling may cause breakages. Bad stacking may lead to distortion.
    • WACQ, WCCA and WCuAz, if not fixated yet, may react with metals.
    • After treatment with WCCA, WACQ, WCuAz and WB preservatives, freshly treated timber should be protected from rainfall and direct sunlight until the surface is dry, especially for high retention end uses. This usually takes about 48h.
    • Class C treated timber, e.g. poles, should be used as soon as possible to prevent excessive migration of the preservative. When stored and horizontally stacked, the timber should be rotated axially once every six months, and direct exposure to sunlight should be avoided.
    • Machining (drilling, sawing, etc.) of treated timber should be minimised as treated zones may be removed or untreated zones inside treated material may be exposed. A similar preservative (excluding WCCA) or suitable and registered remedial brush on preservative should be applied to any freshly exposed surfaces, e.g. cross-cut ends of treated gum poles.
    • Timber treated with Class O preservatives is no more flammable than untreated timber after the solvent has evaporated.
    • Treated timber waste and off-cuts cannot be used as firewood or for cooking purposes and should not be disposed of by burning. Instead, it should be disposed of at a suitable waste disposal site or landfill.

    Fixation of Class W preservatives

    Copper, Chromium and Arsenic (CCA) components become chemically fixed to (or react with) the wood after treatment.

    • Due to the toxicity of hexavalent chromium (Cr6+), freshly treated timber should not be dispatched before fixation of WCCA preservative has been reached.
    • Fixation of WCCA preservatives is time and temperature dependent. Freshly treated timber should be stacked until the fixation is complete before dispatch. This is assumed to be complete when the conversion from chromium VI (Cr6+) to chromium III (Cr3+) is achieved.

    Penetration and net retention

    • The quality of treatment is measured using two criteria: retention and penetration.
    • Treated timber products used in different applications and different hazard classes require different amounts of preservatives, i.e. retentions. They are normally expressed in preservative mass (kg)/timber volume (m3).
    • Due to differences in permeability, different depths of penetration into the timber product can be expected. Minimum depths (mm), i.e. minimum penetration, required per timber product are also specified.
    • Both values are listed in SANS 1288 for different hazard classes and timber products.

    Safe use of treated timber

    Recommendations for the safe use of preservative-treated timber in specific applications are given in SANS 10005, e.g.

    • Direct contact with food for human consumption
    • Direct contact with feeds for livestock
    • Indirect contact with food or feeds
    • Human contact
    • Animal contact
    • Periodic direct contact
    • Contact with plants
    • In contact with plated sheet metal.


    SANS 1288 – Preservative Treated Timber

    Where SANS 10005 primarily focusses on the preservative treatment process to ensure that acceptable preservatives are applied correctly using suitable and correct methods of preparation, handling, impregnation of the timber and post treatment operations, suitable for the designated applications (hazard classes), SANS 1288 specifies the requirements of the treated product, i.e. the required amounts (retentions) and acceptable penetration of the preservative chosen for those products. For example, when treated poles are designed to be used in a structural capacity in a retaining wall under marine conditions (H6), such poles shall be produced according to either SANS 457-2 or SANS 457-3 and have retentions of 24 kg/mCCA plus 200 kg/mcreosote and a minimum penetration of 50 mm (note redrying down to at least FSP is required between the CCA and creosote treatments).  The metal disc attached to the pole indicates that it is a SANS 457-2 or SANS 457-3 pole. It also identifies the treater and states the hazard class (H6, implying the preservative expected retentions) and year of treatment, indicated with the last two digits, e.g. 21 (2021). Table 1 in SANS 1288, seven pages long, lists the H2 to H6 hazard classes, timber applications, end use, preservative type, retentions and minimum penetrations for a range of softwood and hardwood products. SANS 1288, therefore, sets nationally accepted levels of preservative treatment of timber for a classified range of hazard conditions.

    As such, the standard promotes and extends the use of timber and to conserve the raw material. A modified extract of Table 1 appears in Table 7.

    As mentioned earlier, SANS1288 does not imply that only preservative treated timber should be used. Treated timber may not be necessary in low hazard conditions or when the heartwood of naturally durable species is used.

    In SANS1288 it is stated that timber in ground contact (H4) and exterior above ground exposure (H3), should have an expected life span of a least 20 years when treated in accordance with the requirements applicable to the exposure classes. However, it should be noted that conversion of any kind, reshaping or worked in any way after treatment, will reduce the expected service life of the timber. Also, if the timber was unsuitably stored, transported or handled after dispatch from the factory, it cannot be assumed to comply anymore.

    For structural specification purposes, important knowledge of the content of the other standards is necessary. The most important of these are:

    SANS 457-2, Wooden poles, droppers, guardrail posts – Part 2: Softwood species.

    SANS 457-3, Wooden poles, droppers, guardrail posts – Part 2: Hardwood species.

    SANS 753, Pine poles, cross arms and spacers for power distribution, telephone systems and street lighting.

    SANS 754, Eucalyptus poles, cross arms and spacers for power distribution and communication systems.

    Table 7: Hazard classes, moisture contents, timber applications and degrading agents in South Africa


    • Information given in this table is not complete and the reader is advised to consult the tables given in SANS 1288 for more accurate detail and correct interpretation and applications
    • Classification focus: On biological agents and their environmental requirements
    • Classification criteria: Water: type: sea/estuarine/fresh; wet; weathering; leaching
    • Ground/soil
    • Time: constantly/periodically; direct or not (never)
    • Some products can be exposed to one or more hazard classes simultaneously, e.g. H3/H4 or H3/H5 or H3/H6 poles.
    • Compared to the European classification, H3 is not subdivided.


    Bas.: Basidiomycota (brownrot, whiterot); Asc.: Ascomycota (softrot); Bact.: Bacteria;

    Hylo.: Hylotrupes bajulus; Lyct.: Lyctus brunneus; Anob.: Anobium punctatum; Cryp.: Cryptotermes brevis;

    SubtT.: Subterranean termites, e.g. Macrotermes natalensisOdontotermes badiusOndototermes latericius.;

    GeoLo.: Geographical location specific.

      4.2 Modified wood

      Although the modification of wood properties to improve the wood water relations by, e.g,. reducing the hygroscopicity of wood or by improving insect and fungus resistance, has been receiving much scientific interest for over 60 years, environmental concerns regarding the use of certain classes of preservatives have again prompted commercial interest and progress in this more eco-friendly method of wood protection. At the end of the service life it is possible to dispose of such material without presenting an environmental hazard any greater than that associated with the disposal of unmodified wood.

      Methods that have been applied and developed in technologies include:

      • heating wood to temperatures above 200˚C for a certain period, thereby allowing the hydroxyl groups of adjacent lying polysaccharides in the cell wall to react with one another. The nett effect is a reduction in hygroscopicity, i.e. a dimensional more stable material, and a higher fungus resistance also.
      • Impregnation of wood with monomers or prepolymers and curing them. Depending on molecular size, penetration into and polymerisation in the cell wall is possible. Most cell lumen are filled. Impregnation with suitable waxes or oils under elevated temperatures is also possible.
      • Impregnation with chemicals that can react with the polysaccharidic hydroxyl groups, also modifying the chemical and physical properties of the cell wall, is another way in which durable, non-toxic timber is produced.

      As with all methods, physical and/or chemical, some wood properties other than a lower hygroscopic or less attractive material to organisms, may be negatively affected. Most experimentation and technology developments have therefore focussed on minimising the possible impact on especially strength properties.

      Three technologies, viz. heat treatment, furfurylation and acetylation have reached commercial status in Europe and North America (Table 8). Like treatment of timber with preservatives, extensive testing of the modified products is continuously undertaken. Wood species, the influence of their growth conditions, many types of applications, a diversity of environments, etc. are evaluated to provide estimates of their performance. At present, product quality is controlled by control over the process. Standards have not been formulated. It is expected that these products will be available, or produced under license in SA, using our own raw materials. Research into the performance of these products exposed to various SA conditions should be conducted to ensure the success of these new products and associated technologies for local use.

      Table 8: Current commercially available modified products

      The following links to websites can provide some further background and information on some of these products:





      Photo 1: Accoya© Traffic bridge installed in Sneek, The Netherlands.









      4.3 Surface finishes

      The application of a water repellent surface finish/sealer on preservative treated timber or the incorporation of water repellent additives into preservative formulations, is often mentioned, especially when the use of Class O preservatives is considered. The function of water repellent finishes is self-explanatory, i.e. to exercise control over the MC levels of/in the timber product and protect against weathering caused by climatic or atmospheric exposure and variances. Paintability of preservative treated and modified wood products is also an important characteristic of such products. In addition to its water regulating function, surface finishes make an aesthetic contribution, bringing out the “character” such as grain patterns and natural colour of the wood, and/or giving colour to the timber.

      It is a well-known fact that the end of the service life of a CCA treated product is accelerated by the continuous swelling and shrinkage of the timber resulting from consistent wetting and drying during direct exposure to rain, dew, fog, melting snow, etc. and heat from sunlight.

      When finishes are used on exterior doors and windows, balustrades, etc. manufactured from untreated or non-durable wood such as sapwood, it is important that MC levels should be controlled < 20% to prevent potential fungal development and consequent wood degradation. It may happen that design, manufacturing and construction defects cause water to accumulate in the finished product that could lead to fungal attack.

      Choosing a suitable finish can be both subjective and objective: a film forming formulation such as a varnish normally lasts longer than a penetrating sealer (Figure 37). However, maintaining or renovating the film forming finish requires more effort, whereas a penetrating finish requires less surface preparation before reapplication of a maintenance coat. When exposed under exterior conditions, pigmented finishes that is brownish, perform better than clear finishes because it provides added protection against UV degradation caused by sunlight exposure.

      Figure 37: Film forming and penetrating finish types. Distribution of the dry finish on the surface and in the peripheral surface of the wood, respectively.

      Although it is a matter of taste to specify a “rustic” or weathered appearance (Photo 2) vs a more well maintained/looked after finish, some protection of the treated timber surface is recommended. During wetting, exposed end grain will absorb moisture faster and deeper than side grain but when drying conditions set in, it releases water vapour quite easily. Delamination of laminated products can occur and checking of timber at the end grain, especially close to metal fasteners, can lead to structural damage. See Photos 3, 4 and 5.

      Photo 2: Uncoated weathered hardwood gate. Photo: Tim Rypstra

      Photo 3: Uncoated end grain of preservative-treated structural members. Photo: Tim Rypstra

      Photo 4: Delamination due to penetration of rain into end grain (top) and side grain (bottom) Photo: Tim Rypstra

      More detailed information is available from the website of the South African Wood Preservers Association at https://sawpa.co.za/wp-content/uploads/2021/12/FINAL-EXTERIOR-WOOD-FINISHES-high-res-changes.pdf

      4.4 The use of preservative-treated timber in specific areas of SA

      Worldwide, the economic impact of the timber in structures being attacked and damaged by wood destroying agents is huge. Not only does premature failure of timber structures cause financial losses, off-time, inconvenience, etc., but the damage can cause serious injury and possible death. The escalating impact of insect attack by wood boring beetles and wood destroying termites, and to also protect SA grown timber resources, made it necessary for government to introduce legal measures applicable to the use of structural timber in permanent buildings.

       As quoted from SANS 10005:

      “To qualify as being adequate for its purpose, structural timber of the two main species used in a permanent building in the areas given in 12.2 and 12.3 (designated geographical areas applicable to softwoods and hardwoods) shall be preservative treated in accordance with 12.4. (Reference made to SANS 1288 in 12.4 of SANS 10005)

      Gymnospermae (coniferous/softwood species)

      • Sawn timber (including planed and profiled timber) and poles or logs (round or partly round) of the softwood species shall be treated in accordance with SANS1288 when used in the designated municipal areas or towns in South Africa.
      • Sawn timber (used in the erection of an exposed loadbearing structure, i.e. the substructure of decks, shall be treated in accordance with SANS 1288 when used within the borders of South Africa.

       Angiospermae (broadleaved species)

      • All sawn timber, planed timber, and poles or logs (round or partly round) of the hardwood species shall be treated in accordance with SANS 1288 when used within the borders of South Africa. The following products may be excluded if there is insufficient sapwood to obtain the stipulated retention requirements:
        • laminate timber
        • block and strip flooring
        • ceiling board;
        • panelling
        • mouldings and joinery
        • garden furniture
        • outdoor decking boards; and
        • non-sapwood-containing, kiln-dried and planed, sawn boards processed from eucalyptus species.”

      The list of designated municipal areas or towns in South Africa and their geographical location appears in SANS 10005.

      4.5 Legal aspects

      Questions with regards to the legal aspects concerning compulsory timber treatment in South Africa are not discussed in this short course. However, a copy of a document compiled by SAWPA, and which can be downloaded from the SAWPA website, appears below. Further information is available from the Executive Director.


      4.6 Quiz 4

      1. The recommended moisture content of poles allowed for treatment with CCA, at a preservation plant, should be
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      2. The criterium used to monitor SABS approved preservative treatment of sawn boards is:
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      3. The following are SABS approved Class O preservatives:
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      4. The SABS standard that prescribes the process of how timber must be treated with chemical preservatives is
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      5. The presence of the following defects and/or conditions are allowed when timber is treated with chemical preservatives:
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      6. The following wood material can easily be treated with waterborne preservatives:
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      Your First Name
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      5. Summary

      To specify and control the use of wood products for outdoor applications today and in the future, and to understand the application of national and international standards, require elative and specific background knowledge.

      With these objectives in mind, this short course was designed to introduce the following relevant topics:
      • wood properties
      • agents that can degrade wood in service and how these processes take place
      • a description of the major classes of exterior, in-service environments
      • wood protection systems
      • legal aspects concerned with the use of treated timber products in South Africa.

      However, the introductory nature of the learning material still requires an intimate knowledge of the relevant standards referred to as well as of the information material available on the SAWPA website at https://sawpa.co.za

      6. References


      American Wood Preservers Association Standard U1-20. (2020) Use category System: User specification for treated wood.
      Bravery, AF, Berry, RW, Carey, JK, Cooper, DE. (1987) Recognising wood rot and insect damage in buildings. Building Research Establishment, Princes Risborough, Aylesbury, Bucks, United Kingdom.
      Brischke, C, Rapp, AO (2008) Dose–response relationships between wood moisture content, wood temperature and fungal decay determined for 23 European field test sites. Wood Sci Technol. 42:507–518
      BS EN 335 (2013) Durability of wood and wood-based products – Use classes: definitions, application to solid wood and wood-based products. BSI Standards Publication.
      Carll, CG. (2009) Decay hazard (Scheffer) index values calculated from 1971-2000 climate normal data. FPL-GTR-179.
      Conradie, DCU. (2012) South Africa’s climatic zones: Today, Tomorrow. International Green Building Conference and Exhibition. July 25-26, Sandton, RSA.
      EN 350 (2016) Durability of wood and wood-based products – Testing and classification of the durability to biological agents of wood and wood-based products. CEN-CENELEC Management Centre, Brussels.
      Haygreen, JG, Bowyer, JL (1982) Forest Products and Wood Science. The Iowa State University Press, Iowa, USA.
      Rypstra, T. (1995) Analytical techniques for evaluation of wood and wood finishes during weathering. PhD dissertation, Stellenbosch University, Stellenbosch.
      SANS 10005, The preservative treatment of timber. SABS, Pretoria.
      SANS 1288, Preservative-treated timber. SABS, Pretoria.
      SANS 457-2, Wooden poles, droppers, guardrail posts – Part 2: Softwood species.
      SANS 457-3, Wooden poles, droppers, guardrail posts – Part 3: Hardwood species.
      SANS 753, Pine poles, cross arms and spacers for power distribution, telephone systems and street lighting.
      SANS 754, Eucalyptus poles, cross arms and spacers for power distribution and communication systems.
      Simpson, W. (1998) Equilibrium Moisture Content of Wood in Outdoor Locations in the United States and Worldwide. Research Note FPL-RN-0268.
      Schultz, TP, Militz, H, Freeman, MH, Goodell, B, Nicholas, DD. (2008) Development of Commercial Wood Preservatives. ACS Symposium series 982. American Chemical Society, Washington, USA.
      Siau, JF. (1995) Wood: influence of moisture on physical properties. Dept of Wood Science and Forest Products. Virginia Polytechnic Institute and State University, USA.
      The Timber Preservers Association of Australia. (2016) Technical Note 2. Understanding Hazard Classes.
      Van Vuuren, NJJ, Banks, CH, Stöhr. Shrinkage and density of timbers used in the Republic of South Africa. Bulletin 57. SA Forestry Institute, Pretoria.
      Wang, C-H, Leicester, RH, Nguyen, MN (2007) Manual No. 3. Decay in Ground Contact USP2007/040, Forest and Wood Products Australia.
      Wang, C-H, Leicester, RH, Nguyen, MN (2007) Manual No. 4. Decay above Ground. USP2007/040, Forest and Wood Products Australia.
      Wilkinson, JG. (1979) Industrial timber preservation. Associated Business Press, London.
      Winstanley, JK, Cillie, JJ. Identification of timber damage. Plant Protection Research Institute, Rosebank 7700. South African Lumber Millers Association.

      Date: 1 March 2022
      Author contact details:
      Tim Rypstra
      Email: tr@sun.ac.za, Mobile: +27 8 334 84 334

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