Do not let the bugs bite

When we think of protecting timber, weather-proofing is often the first thing that springs to mind. However, an important consideration to prevent compromised performance in timber is protecting susceptible timbers and wood-based products against wood-destroying organisms such as fungi and insects. 

Suitable preservative treatment and/or relevant surface and edge coating, and proper maintenance and installation, are key to ensuring good levels of protection. This begins with understanding the use classes of solid wood and wood-based products before determining preservative treatments. The use classes range from 1 to 5 with level 1 referring to dry interior timber and level 5 being permanently or regularly submerged in salt water. In this blog, we consider important parameters vital to gauging the type and level of treatment required. 

In-service conditions

When specifying preservative treatment, it is first important to consider any conditions that could arise during the service life of the component, which would result in unexpected wetting of the timber. For example, design faults, condensation, failure of other materials, poor workmanship or lack of maintenance may suggest assigning a more severe use class. This would, in turn, influence the level of treatment required. 

Moisture and insects

For fungal decay to occur, timber must be damp with a moisture content exceeding 20%. Therefore, timber of any species that is maintained in dry conditions (use class 1) will remain free from fungal decay. However, for timber components exposed in use classes 2 to 5, the life of timber components can be extended considerably by the designer’s attention to detail, for example, by protecting against excessive wetting and avoiding moisture build-up.
In use class 1, the only hazard is insect attack. But insects can also affect durability in higher use classes. The consequences of attack are slow to become apparent in most cases, but take account of the risk over the lifetime of a component. 

Natural durability

Timber species vary in their resistance to attack by wood-destroying fungi and insects. Timbers of commercial importance in Europe can be classified to natural durability classes against decay agents with 5 classifications from 1 (very durable) to 5 (not durable). These classes refer to the heartwood and resistance to fungal decay.

Many of the naturally durable timber species originate in the tropics and may not be available certified. In these cases, it may be more appropriate to specify a less-durable certified timber with preservative treatment. If the heartwood of a timber has sufficient natural durability it can be used without preservative treatment, even where a recognised biological hazard exists. 


In the UK, wood preservative products are approved under the Control of Pesticides Regulations 1986 (COPR, as amended 1997) or the EU Biocidal Products Regulations (528/2012).

In addition to the requirements laid down in UK building regulations, there are a number of organisations concerned with the performance of timber commodities that specify requirements for the treatment of timber. These include the housing warranty bodies.


Documentation should accompany preservative-treated wood and treatment certificates provide evidence of the:

  • type of preservative applied to timber
  • method of application
  • intended use class of the treated product
  • company that applied the treatment

These certificates provide the customer with evidence of treatment, especially when the treatment does not change the appearance of the wood. 

With a multitude of things to consider when protecting your timber, enlisting the help of a timber consultant from BM TRADA can help alleviate the stress of your project and ensure that you have world-class support. 


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