Summary
• There are five preservative treatment Use Classes.
• A WPA manual sets out which preservative treatments are suitable.
• Good design and maintenance are essential in achieving the desired service life.

For many end uses, the natural durability of commercial softwoods and some hardwoods is insufficient to provide confidence in performance in service. For such timbers, durability can be conferred by preservative treatment, or some other form of wood modification that improves resistance to decay or insect attack.

Preservative treatment is tailored to the risk timber faces in five Use Classes: interior dry; interior – risk of wetting; exposed to weather, above ground (3.1 coated/3.2 uncoated); in ground or water contact; in sea water.

The Wood Protection Association (WPA) manual – Industrial Wood Preservation – Specification and Practice – includes a range of commodity specifications that set out what level of natural durability is required and, if timbers of otherwise inadequate durability are preferred, which preservative treatments are suitable for different timber commodities in the various Use Classes for typical or recommended service lives. The specifications are based on BS 8417 recommendations, adding helpful guidance for specifiers.

For example, the typical desired service life for fencing is 15 years. To achieve this, the specification requires all cutting, drilling, profiling and shaping of the timber to be carried out before treatment. Where on-site cutting cannot be avoided, freshly-exposed surfaces must be re-treated with a product recommended by the pre-treatment preservative supplier.

Good design and maintenance also play an essential part in achieving the desired service life; preservation is not a substitute for good practice in these areas, rather it is part of best practice procedures.

Confidence in preservatives

Why do we have such confidence in the performance of timber treated with preservatives supplied by WPA members?

Preservatives are supplied by companies serving an international market and who have strong reputations for excellence in research and development both in the UK and overseas. The companies involved have spent years evaluating the performance of new preservative formulations, not only because the future for the older types such as CCA was in doubt, but because innovation has long been a driver in this industry.

Preservative development involves teams of chemists and wood preservation specialists testing formulations first in the laboratory using well-established standard tests, then, for relevant Use Classes, with long-term field trials. The latter allow predictions to be made of service life of full size timbers, though the final proof comes from performance in service.

To continue the fencing example, for above ground parts to be painted or protected from the weather, all of the current commercial preservatives, including the latest copper-organic types, are suitable (except creosote which is not compatible with surface coatings). For timber to be used without a coating, typically copper-organic or creosote (where permitted) can be selected for both above ground and in-ground components.

Specification and fit for purpose

The service life indicated in standards like BS 8417 can only be fulfilled if treatment conforms to the required combination of penetration and retention of preservative.

Where the specification lacks detail, it is perhaps easy to feel that the customer gets what he deserves. Specifications such as ‘green treated’ convey no information about the Use Class or the required service life. On the other hand, a specification based on the WPA’s commodity specifications provides all the information required to enable the treater or the merchant to supply appropriately treated timber.

We certainly want to see more and better specifications being used but treatment companies and suppliers of treated timber must also accept responsibility as experts in their field to seek the necessary information from the customer and provide product that is fit for purpose. Approved treater members of WPA have the skills to deliver treatment to the required standard.

It may be that a 15-year service life (the minimum in BS 8417 and the WPA commodity specifications) would meet a fitness for purpose test in court; but timber treated, for example, to a lower Use Class than is required and which performs poorly in service is unlikely to be regarded as fit for purpose by any reasonable person.

The customer or installer has obligations too – basic good practice techniques must be followed, including avoiding cutting, boring or notching on site.