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Wood rot the "forgotten" challenge

wood timber rot

Wood decay/rot is a phenomenon that can affect timber irrespective of the type involved or the use to which it is put.

Moreover, due to the heavy rains of late, the threat of wood rot has reared its ugly head to an unusually large degree – a factor which stakeholders in the timber sector from industry technophiles to consumers, need to take into account.

Basically, organisms affecting wood have specific requirements for survival: food, moisture (although some fungi that attack wood are able to carry their own moisture source with them), oxygen and a suitable temperature.

Wood is a food source that has the ability to hold moisture to varying degrees and it’s therefore at risk of attack by a range of organisms. In turn, the types of organism involved depend upon many factors; for example, the timber species and end-use.

Dealing more specifically with the issue of rot, there’s a need to distinguish between stain and mould and fungal decay.

Staining fungi

Attack by staining fungi results in discolouration of the timber, but the damage is purely superficial – the strength of the timber is not affected although where timber appearance is important, its value can be significantly reduced by this kind of attack.

Staining fungi can attack under conditions of high humidity and /or where the timber is freshly felled and contains plenty of nutritious sugars.

Where infection occurs, the stain is almost invariably associated with superficial discolouration caused by moulds forming greenish or black, occasionally yellow, powdery growths. The good news is that moulds can be brushed away and re-infection prevented by removing the source of moisture.

Wood Rotting fungi

Wood rotting fungi, on the other hand, seriously weaken the timber and may render it quite valueless. Damage can occur in buildings and in outdoor situations. Timber is at risk whether it’s above ground or in-ground contact. Fungi types are subdivided into three groups:

  • Brown rots: so-called because they feed on the lighter coloured cellulose content of the cell, and leave the darker lignin more or less intact. This type of rot is often characterised by breaking down of the cell wall structure which results in crumbling and the timber becoming powdery in texture. An example is dry rot, which can cause serious damage in buildings.
  • White rots: so-called because they feed on both lignin and cellulose, rendering the timber lighter in colour. This type of rot is often characterised by small white pockets and causes the timber to become brittle in texture.
  • Soft rot: these are prevalent in very wet environments, such as soil. They soften the exposed surfaces and gradually progress to increasing depths causing shrinkage as the outer layers decay.


Treatment with a wood preservative such as Tanalith C (CCA) plays an important role in reducing exposure to wood decay. The chemistry in manufacturing such a product offers dual protection – against insect attack and fungal decay.

Fungal protection is provided by metal-based ingredients such as copper oxide. However, certain products also offer recycled metal-based ingredients (e.g. Tanalith E) and there are products that use metal-free chemistry as well – replacing the metal-based ingredients with specialised biocide formulations which are organic and are thus biodegradable.

The hazard factor

In treating timber for rot (and other agencies as well, of course) a hazard class classification system is used to identify the level of risk to which the timber in use is subjected to.

As one moves up the hazard classes, the greater the risk of fungal decay. For example, H2 exposure requires only mild fungal protection, H3 slightly more fungal protection and so on, up to H4, H5 and even H6 levels of severe hazard decay.


Notionally, of course, consumers and experts alike are all aware of the wood rot factor. But perhaps in reality, the impact of rot is perhaps sometimes under-rated.

Consider, for example, the integrity of roofing against heavy rain downpours. Rainwater can flow back under roof eaves or gain access through damaged roof tiles and sheeting or through leaky roof caps (particularly if gutters are clogged or damaged).

With repeated water contact, conditions for dry rot are created which can ultimately result in extensive damage to truss ends, barge board, fascias, purlins and brandering. The use of Tanalised preservative treated timber for roofing structures provides the extra assurance that a roof is protected, irrespective of geographic area.


Lonza Wood Protection

16 Indus Road, Marburg
Port Shepstone
South Africa

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