THE REAL WOOD GUIDE TIMBER TREATMENTS

NATURAL DURABILITY AND PRESERVATION

In our everyday lives we are surrounded by examples of wood which are centuries old. Britain's towns, villages and farms contain an enormous array of wood which has given good service for fifty years and more. Our coast and our rivers and our countryside show wood being routinely used in tough exposed places.

Wood has enemies. Among them are fungal decay, woodworm and marine borer, but they can be kept at bay.

Understanding the effects of the environment in which the wood will be used is important. It will dictate choice of wood product and whether additional protective substances have to be used. Designing to avoid build up of persistant damp is important.

FUNGAL DECAY

Fungal attack, which we know as rot and decay, can only occur when the fungus has a source of moisture and can obtain oxygen from the air. The simplest way to avoid decay is to keep timber dry. Timber with less than 20% moisture content will not decay because it is too dry. Even the so called ‘dry rot' has to have a source of moisture. In our homes and offices the moisture content will be less than 20% and where there is central heating the moisture content of timber is likely to be 8% or even lower. Of course if there are roof leaks, or water seepage from pipes or from other sources then there could be trouble. This is a matter of good maintenance.

Wood, once it has been dried from its original fresh green state, behaves in balance with the moisture content of the air around it. It takes up moisture and tends to swell when the atmosphere around has higher moisture content, and gives up moisture and tends to shrink when the air becomes drier. This is known as "movement', with each species having its own characteristics.

Well designed structures which prevent rain from accumulating on surfaces or in joints ensure long life. Wood which is exposed to damp and rain can be given extra protection with wood preservatives or with paint, varnish and other water repelling treatments.

Protective films like paint and varnishes which are to be used on wood which is exposed to the elements and to water have to be carefully applied to ensure that all surfaces are completely sealed -particularly the more porous end grain of timber, joints and cut-outs. The aim is to avoid trapping excessive water below protective films. Paint and varnish manufacturers' instructions must be meticulously followed.

Water repellant preservative systems which allow the material to breathe usually incorporate decorative colour stains. Raw timber exposed to sunlight and the weather usually loses its natural colour and eventually becomes a pleasant silver grey. This is nothing to do with rot. The combination of water repellancy, preservative and stain in a single treatment helps keep the original colour if that is what is wanted.

NATURALLY LONG LASTING

Natural resistance to fungal decay varies from species to species. Some species, in particular some tropical woods, have heartwood of very high natural durability, which is why they are specially selected for tough outdoor use in sea defence, decking and structures.

Where high natural durability is required, it is normal to exclude timber containing sapwood as it is generally less durable. The sapwood is the narrow outer band of wood immediately below the bark - it is the living wood where the sap flows. Sapwood is just as strong as the heartwood. Its lower resistance to decay is not a problem for internal dry conditions. It is more easily penetrated by wood preservative.

INSECT ATTACK

Woodworm is a non technical description for the damage caused by certain insects and their larvae.

Insect attack is mostly something which the forester, the logger and the timber processor have to worry about. There are pests which infect the standing tree or freshly felled 'green' timber but the industry has remedies for these.

For the consumer the insects of most concern in the UK are Anobium punctatum (furniture beetle), Xestobium rufovillosum (death watch beetle), and to a limited extent in certain parts of the country only, Hylotrupes bajulus (house longhorn).

For many uses, where the wood can be seen every day, incidence of attack can be spotted. The appearance of holes on the surface is the result of activity starting from the hatching out of insect eggs three to five years earlier. Surface varnishes and polishes considerably reduce the risk of attack if they prevent the insect laying its eggs in the wood. And modern levels of heating create dry atmosphere which insects dislike. Risk of insect attack is increased by the presence of persistent damp.

USE OF PRESERVATIVES

Specific preservation treatments can also be used to provide additional protection against fungal decay and insect attack.

Preservatives vary in chemical make-up and the ways in which they are applied. There is no doubt that the safest and most thorough forms of treatment are those carried out by the manufacturer using impregnation under pressure in special equipment to ensure deepest penetration of preservatives and also to ensure that the wood is safe to handle and use after treatment. It is commonplace now for window frames and roof elements to be pre-treated. Pressure impregnation is governed by regulations to ensure safety for operatives and the users of the treated wood. The British Wood Preserving & Damp-proofing Association (BWPDA) can recommend qualified operators and information about types of preservatives and methods of application. If you do it yourself you must follow the preservative manufacturer's instructions to the letter. Manufacturers are obliged by law to print safety instructions on the labels used on preservative containers. Preservatives, by their very nature, are toxic substances and must be handled extremely carefully, especially in closed surroundings where the spread of preservatives and their solvents into the air can pose a health risk, unless the proper precautions are taken.

There are, at present, differing opinions as to the acceptability of certain chemicals used in wood preservatives. The timber trade does not manufacture wood preservatives (they are produced by the chemical industry) but supports any activity, whether in preservation or other fields, which enhances safety for the user. Experts in health and safety are reviewing the situation and there are moves to see that controls over the use of wood preservatives should be standardised throughout the European Community.

The British Wood Preserving & Damp-proofing Association says that all wood preservatives legally offered for sale in the UK must be approved under Control of Pesticides Regulations 1988 and that these regulations embody the requirements of the Food and Environment Protection Act 1985, the Classification, Packaging and Labelling of Dangerous Substances Regulations 1984, and the Wildlife and Countryside Act 1981, as well as the broader implications of the Health and Safety at Work Act 1974. The regulations control the composition of the products, the category of use, the wording on the label in respect of hazard category, risk phrases and safety phrases, and regulates the storage, handling, transportation and use of wood preservatives at all stages.

British Wood Preserving and Damp-proofing
Association
6 The Office Village
4 Romford Road
Stratford
London El5 4EA

Next Section: 'Non Timber Resources from the Forest'