British Standards for wood preservation have been published since at least the 1950s, but they have not been well understood outside the immediate wood preservation industry, least of all perhaps by specifiers.

Persuading the latter to use precise and unambiguous specifications for treatment has been a long struggle, not helped by the regular changes inevitable with standards in an industry increasingly regulated on health and safety (eg the Biocidal Products Directive) and performance (the Construction Products Directive). The latter directive drove the industry at European level to agree by the late 1990s a raft of standards setting out the basis for assessing the effi-cacy of preservatives and a system for defining treated timber based on preservative penetration and retention. Most of these many standards are only of interest to preservative manufacturers who use the standardised test methods to assess efficacy.

The new European system had to be interpreted for UK use. For specifiers and treaters the standard that does this, and is in fact the only one they need to refer to, is BS 8417:2003 Preservation of timber. Recommendations. This sets out recommended treatment levels for all end uses for the commonly-used preservative types. It has recently undergone a review and a revised edition will appear probably during 2007. Furthermore, the European standard that established a classification of timber uses, called Hazard Classes, based on the risk of wetting in service, has also just been revised and the Hazard Classes are now to be referred to as Use Classes. Is it any wonder there’s confusion?

Treatment tailored to risk

So treatment is now tailored to the risk faced by timber in five Use Classes – 1: interior dry; 2: interior – risk of wetting; 3: exposed to weather, above ground (now two sub-classes); 4: in ground or water contact (now two sub-classes); 5; in sea water. This is a useful classification reflecting an increased risk of (mainly) fungal decay as wetting risk increases, ending in timber in the sea where there is the additional risk of marine borers (shellfish that tunnel into wood near the water surface).The risk of attack of timber by wood-boring insects is largely independent of wetting and has to be taken into account separately.

BS 8417 defines treatment to Use Class but also uses service factors that, as well as risk of attack, take account of the consequences of attack in relation to cost of repair and potential risk to safety in use.

The Wood Protection Association has a strong tradition of providing simplified but accurate guidance through the potential minefield of treatment specification and is shortly to publish an update of its preservation manual that provides a straightforward route to specification taking all the details into account. An example of such a specification is Fencing timber (Use Class 4) treated in accordance with WPA specification C3. While appearing a very simple specification, this incorporates all the requirements of BS 8417 and thus all the relevant European Standards and provides the treater with all the information he needs to carry out a fit for purpose treatment.