23 September 2021

How do you specify preservative-treated wood?

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Member feedback from the Timber Trade Federation (TTF) and Wood Protection Association (WPA) suggests that meaningless generic expressions such as ‘pressure-treated’ and ‘green-treated’ are unfortunately still common. The inaccurate or vague description of preservative-treated wood products inevitably increases the chance of failure in service and subsequent customer complaints, particularly for outdoor applications. 


This inaccuracy risks damaging the reputation of timber as a reliable and versatile building material, undermining the opportunities to build confidence and grow the demand for treated wood. Additionally, consumer and contract law require that any product offered for sale must be fit for its intended use and a wood product impregnated with preservative is no exception.


Accurate specification and description

The WPA and TTF work in partnership on all wood protection matters, and both associations are now focused on one clear priority on behalf of members – to promote the accurate and unambiguous specification and description of treated wood products throughout the UK supply chain.


Staff turnover at some merchant outlets and the sheer range of products now handled by many merchants makes the consistent enhancing of product knowledge on one particular material a challenge. The WPA believes that the answer is to keep the message as simple as possible, communicate it through the right channels and keep reinforcing it until it becomes standard practice for both buyers and sellers. The partnership between TTF and WPA, further supported by the Timber Decking and Cladding Association (TDCA), is key to ensuring consistent messaging and maximising the audience reached.


Use classes

It is a mistake to assume that all pressure-treated wood is the same. While one piece of treated wood may look much like any other, the level of preservative protection could be quite different. The British Standard for wood preservation, BS 8417, requires that the loading and penetration of preservative impregnated into the wood is tailored to the desired end use. Applications for treated wood are therefore grouped into ‘Use Classes’.


The challenge is how best to simplify this message while not losing technical accuracy. All three associations have been working together on a two-fold approach, in line with the target markets:

  • For the fencing, garden and landscaping sector (i.e. for treated wood used in contact with or close to the ground), the focus is on promoting the use of the term Use Class 4 primarily through the WPA’s ‘Make sure it’s Use Class 4’ logo and communications package.
  • For the broader timber supply chain, the focus also includes differentiating between interior and exterior applications for treated wood – using a bold, colour coded Use Class 2, 3 or 4? message and simplified product descriptions.


Specification and installation checklist

  • Establish the Use Class of the timber you need before ordering.
  • Tell your supplier, in writing, that the wood must be treated to that particular Use Class to comply with BS 8417. Ask them to verify that the wood supplied meets your Use Class specification on the delivery note and invoice or by providing a treatment certificate.
  • When buying from stock always check to which Use Class the wood has been treated.
  • NEVER substitute wood that has been treated for an indoor application for use in an external application – failure is inevitable.
  • Do NOT supply wood that has been treated for external use for internal applications.
  • For wood in permanent ground or fresh water contact, or providing exterior structural support, Use Class 4 levels of protection MUST be achieved. Anything less and service life, structural safety and customer satisfaction will be compromised.
  • When cross-cutting, notching or boring treated timber products during installation, ALWAYS apply an end grain preservative treatment to freshly exposed areas – to maintain the integrity of the protection.
  • NEVER put cut ends in the ground, even if the end-grain has been coated.


For those who need further guidance on what the Use Classes mean in practice, WPA and TTF have also developed three new Guidance Notes on Understanding Use Classes 2, 3 and 4 for Preservative Treated Wood. These include a simple explanation of what national standards require for each end use, which species are most suitable, what preservative penetration to specify for different treated commodities and the necessary quality control criteria.


About the author

Gordon Ewbank, Chief Executive Officer, WPA


For those treatment companies who may need more in-depth detail, see the WPA Code of Practice or visit the website thewpa.org.uk


This is an extract from the Timber 2021 Industry Yearbook. Download the full article, which includes further reading and references, here