By Candace Sofianos King | Photos by Creative Commons.
As a natural and renewable building material, timber has undergone maintenance for centuries. What preservation processes are used today and how can correct timber treatment extend the longevity of wooden structures?
Timber is the only structural raw material boasting a truly renewable and sustainable source of supply. There are timber structures in existence after hundreds of years of service, but there are fence posts which have decayed after only 12 to 18 months’ service. Timber is an organic material that can support the life of other organisms if the environment is suited to their growth, and under certain conditions, may lead to rapid breakdown of the wood.
Wood preservation essentially is the science of wood treatment, utilised to increase durability and give extended service life. This involves the placement, within the wood microstructure, of preservative chemicals which are antagonistic to wood-destroying agencies. Wood preservation promotes the conservation of indigenous forests by making commercially grown plantation timbers, which aren’t naturally durable, effective substitutes for durable timbers from indigenous forests. It also reduces the volume of wood used by prolonging the service life of timber for many years.
According to Bruce Breedt, executive director of the South African Wood Preservers Association (SAWPA), it’s necessary to understand the structural form and characteristics of timber and how this varies from species to species as these factors determine the extent to which wood is naturally protected from decay and the ability to penetrate it with protective chemicals.
“Durability of sapwood, irrespective of species, is low. It generally boasts a lower density than Heartwood and has a high moisture and starch content, all of which are conducive to fungal or insect degradation. Wood that is kept dry and is naturally durable is able to resist invasion by fungi or insects. Heartwood contains cells which have chemical deposits within the cells, little or no carbohydrates, and relatively lower moisture content. Heartwood in some species, such as sneezewood, can be highly durable while in others, for example Pine or Eucalyptus saligna/grandis, durability is low,” explains Breedt.
He adds, “Durability of permeable timbers may be increased by the addition of chemicals that are toxic to fungi or insects, or which reduce the possibility that the moisture content of wood will increase from the seasoned state. It is the addition of these chemicals that provides the basis of the timber preservation industry.
“Permeability, the ability to penetrate wood with chemicals resistant to insects or fungi, is largely dependent on being able to remove moisture so that the preservative liquid can be added in its place. Most preservatives require pressure to push them deep into the wood. The depth to which penetration is achieved is dependent on the available Sapwood, density, chemical inclusions within cells, moisture content, cell type, techniques used and so forth. Generally, softwoods are more easily penetrated than hardwoods, and sapwood is more easily penetrated than heartwood.”
Protecting your timber
Depending on the service and exposure conditions, timber can be attacked, degraded and even completely destroyed by one or more wood deterioration factors. Proper design and preservation practice can eliminate or minimise such an attack, which include fungi, wood destroying insects, weathering and fire.
According to TimberLife director Willie Conradie, the development of fungal attack, under favourable conditions, can be rapid. “Wood degrading fungi can be broadly divided into two main groups: wood discolouring fungi (mould and sapstain fungi) and wood rotting fungi (brown rot, white rot and soft rot fungi). Although wood discolouring fungi does not damage the wood structure, they adversely affect the general appearance of the timber,” explains Conradie.
He adds, “Mould fungi produce masses of pigmented spores, usually greenish or black, that cause superficial discolouration of the wood surface. Sapstain fungi on the other hand cause deep and permanent discolouration of the wood. Wood rotting fungi attack the wood itself and can weaken the wood structure to such an extent that it breaks and crumbles away.”
Breedt says fungi develops from minute spores that germinate in suitable conditions and send out filaments called hyphae. These penetrate the wood structure, and if suitable conditions exist, break down the wood tissues into simple chemical compounds on which they feed. Conditions necessary for the development of fungi include moisture content suitable for their development, adequate oxygen supply, a temperature range to suit their life cycle, adequate nutrients and sufficient time.
Most timbers, under certain conditions, may be attacked by wood borers or beetles. Infestation by some wood borers may be of little or no significance, whereas attack by other borers may be serious and necessitate remedial or preventative action. “Wood borers are beetles which at the larval stage of their development bore into wood for food or shelter and tunnel in the timber to derive their nourishment. With some exceptions, the only damage they cause as adult beetles is the cutting of a flight or emergence hole through the surface of the timber as they escape from it. In South Africa the wood borers of main concern can be categorised as the Lyctid, Cerambycid and Anobiid species,” highlights Breedt.
Conradie says virtually all non-durable timbers can be attacked by wood boring insects of one sort or another that may cause extensive damage to the timber. “The most important wood borers are known as the powder-post Beetle, the European house borer and the common furniture beetle. Marine borers attack all non-durable timbers exposed in sea water especially in the warm waters of tropical zones.”
In the case of termites, or white ants, damage is accepted as a significant risk to building and other structural timbers in most parts of South Africa. “Wood destroying termites can cause devastating damage to building and structural timbers. Although drywood termites are mainly restricted to the Durban area, subterranean termites are found virtually throughout South Africa,” notes Conradie.
Wherever there is a risk of termite attack, it is wise to take some precautions. For buildings, these precautions usually take the form of chemically treated-soil barriers or physical barriers and the use of properly preserved treated timber. “There are many varieties of termites and borers encountered in various localities and there appears no reason to think that the areas affected are not spreading. Like bees and some ants, termites are social insects living in colonies which with some species, in a mature colony, may contain large numbers of termites,” says Breedt.
In terms of physical degradation, ultra-violet (UV) radiation present in sunlight has a strong degrading effect on wood, particularly when combined with moisture fluctuations. The periodic movement of moisture into and out of the wood causes more serious breakdown of its structure. As the wood absorbs and releases moisture, it swells and shrinks and results in internal stresses that build up inside the wood structure. Unless the process of swelling and shrinking is inhibited by the correct protection and maintenance thereof, the wood will start to split and crack. This not only adversely affects the aesthetic appearance of timber but may also weaken its structural integrity.
“Protection of timber against natural weathering requires penetrating water repellent wood sealers that have been specifically formulated for exterior applications. In most cases a combination of wood preservation to prevent biological attack and wood protection against weathering is required to fully ensure the long-term durability of timber used under exterior conditions,” says Conradie.
He adds, “Fire is the most destructive enemy of wood and it can completely destroy timber structures within minutes, especially when hot and windy conditions exist. For effective protection and control of potential fire damage and destruction, timber needs to be pressure treated with an acceptable fire retardant product. The destructive nature of fire can only be effectively controlled and slowed down by proper pressure impregnation of timber with an effective fire retardant.”
The various hazards which wood material will be subjected to has enormous bearing on the extent to which wood preservation will be effective, notes Breedt. For example, a piece of wood kept continually dry inside a building is subject to a much lower hazard than a piece embedded in the ground. The hazard level determines the required intensity of the key wood preservation factors, namely the toxicity of the preservative chemical, its fixing characteristics, the penetration required and the retention required.
“For example, interior timbers may only require protection against wood borers, necessitating a simple borer-specific, unfixed chemical-like boron. It also may require penetration only in the outer sapwood, as well as a retention level of a very low order. However, for ground contact hazard, a heavy duty preservative such as a Chromated Copper Arsenate (CCA) or Creosote is required, with continuous penetration to a significant depth and with relatively high retentions,” explains Breedt.
Best practice in wood preservation
The most common and oldest form of wood preservation has been the impregnation process, and more commonly pressure processes, ranging from low to high pressure processes that may or may not include vacuum cycles during the processes. These processes use mainly biocidal active ingredients ranging from heavy industrial-type oil-based preservatives to water-based metallic inorganic preservatives as well as water- or solvent-based organic preservatives.
“Of late there has been some developments – mainly internationally – in modification processes both thermal and chemical (not biocidal) that render timber as a
non-food source for decay fungi and wood destroying insects; however, these modification processes vary in its extent of application and also its impact on the structural and mechanical properties of timber,” says Breedt.
He continues, “The process of traditional wood preservation has not changed that much, apart from a few new adoptions, due to the changes in the preservative types used, with a noticeable move to more environmentally friendlier preservatives. The criticality of end application, conditions and effectiveness of preservatives will, in many cases, determine the types used. The most significant change in methods have been the advent of modification processes as an alternative to pressure impregnation processes; however, these methods are still very much applied in a niche market for now due to availability, price, as well as long-term efficacy in different exposure conditions.
“The effectiveness of all wood preservatives depends on the penetration achieved and the retention of preservative in the penetrated zones of the timber commensurate to the use class, exposure and end application. These are dependant of selecting timber that is permeable and susceptible to the preservation and importantly prepared correctly, seasoned as high moisture content is a limiting and restricting factor when timber is preserved.
“To effectively protect timber against biological attack, wood preservation products and the appropriate application techniques are required. The only way to avoid and totally prevent biological attack on timber is to use timber that has been pre-treated by a proper pressure impregnation or diffusion treatment process with approved wood preservatives such as CCA or borates. However, if not pre-treated with a wood preservative, wood borer, termite and fungal attack on timber can also be effectively controlled by application of brush-on biocides,” says Conradie.
As for the coatings side of timber preservation, newer generation hard wax oil products have really come into their own. “We have seen a huge increase in the demand for hard wax oils versus the usual surface coatings that have been used historically such as varnishes and penetrating oils. This is predominantly due to the maintenance required for both options. It is very important to periodically inspect timbers used externally and maintain them if or when needed,” says Carel Steenkamp, channel manager of Rubio Monocoat South Africa.
Steenkamp says these hard wax oil products will stain, seal and protect timber in a single layer and can also lend a decorative effect as they incorporate colour pigmentation. The pigmentation used can add UV protection to the surface, and as such, slows down cell degradation, extending the life of the timber even further. “Additional protection can be added by using a sun primer – this is a pre-treatment that will enhance the colour, assist as an anti-fungal treatment and strengthen the UV protection already in the oil. Due to its molecular bonding technology, these products will bond to timbers that are CCA treated, as well as thermal treated timbers,” notes Steenkamp.
He adds, “With penetrating oils, the oil is to be applied as soon as the surface appears dry. This will feed the wood but offers little protection against wear and tear. Maintenance requirements may vary, but it could range from monthly to annually. With varnishes, the surfaces need to be lightly sanded and recoated periodically.
“Should the varnish start to flake, the surface should be stripped back to raw wood and re-coated as a new surface. This offers good protection against wear and tear, but the maintenance is intense when due to the sanding required, and the drying times between coats. Maintenance may vary and can range between annual to once every two to three years depending on products, wear and tear as well as environmental factors.”
Conradie says there are numerous advantages in treating timber with boron-based wood preservatives by using the dip diffusion process instead of the conventional pressure impregnation process, especially in the case of structural sawn timber intended for Hazard Class H2 applications.
“Boron-based wood preservatives are substantially cheaper than other waterborne wood preservatives currently available in South Africa. Savings in chemical cost for Hazard Class H2 treatment can be in the order 40% and more. Borates are also the only wood preservatives that can be applied to timber by either conventional pressure impregnation or by simple dip diffusion treatment.”
Taking note of timber preservation hazards
Factors that have bearing on the effectiveness of biological preservation systems:
- The biological hazard to which the wood will be subjected in service.
- The toxicity of the preservative chemical to the particular wood-destroying organisms which will be encountered. Also, the permanence of the preservative chemical under given conditions of hazard following treatment of the wood.
- The penetration and retention of preservative chemical, for example, the extent of the penetration of the preservative chemical into the cross section of the timber and the amount retained in the penetrated zone per cubic metre of the wood.
Factors that determine the effectiveness of preservative treatments against weathering and fire:
- Natural durability of the wood.
- The presence or absence of sapwood.
- Variability within and between pieces and species.
- Preservative distribution gradient.
Timber categories ranging from low to high hazard include:
- Interior timbers – indoor framing, linings and joinery.
- Exterior timbers outdoors above ground – cladding, barge boards, window joinery, palings, rails and bridge decking.
- Ground contact – posts, foundation piles, poles, house stumps, crib walls, landscape timbers, playground equipment, bridge and wharf timbers.
- Timber used in fresh water or heavy wet soil – poles for livestock pens, piling and jetties.
- Marine timbers – marine piles and sea walls.