Key facts on working with wood

Key facts on working with wood

Make sure you know the key facts on wood products and flame-retardant treatments.

If you’re working with an architect or a homeowner, would you be able to recognise when to raise a question on the possible need for timber flame-retardant treatments? It may come as a bit of a shock, but a recent survey of architects found that just over 70 per cent of those surveyed were unable to define what is meant by the term ‘fire resistance’ and just under 60 per cent were unable to explain the term ‘reaction to fire’.

This vocabulary is fundamental to recommending and installing timber systems that are as safe as possible for homeowners and building users. It, therefore, behoves professional builders to understand the basics, as well as the terminology of flame-retardant treatments. The Timber Trade Federation (TTF), and Wood Protection Association (WPA), are working closely together to get correct information into builders’ hands, to help protect your own reputation.

How do flame-retardant treatments work?

Flame-retardant (FR) treatments, applied under factory-controlled conditions and tested for consistent quality, modify the burning process. They cause rapid dehydration of the wood and cooling of its volatile elements, which in turn reduces the rate of growth of the fire. In the early stages of a fire, it is this crucial delay, giving extra time for escape, which is most valuable. Slowing the growth and spread of a fire also provides an opportunity for the flames to be extinguished before extensive damage to a property can be caused, thereby potentially limiting insurance losses.

FR treatments can be applied to products through impregnation in a pressure treatment plant, the same as for normal timber preservative pre-treatments. They can also be added to certain products during their manufacture – products like MDF and OSB for example. There is a third type: the brush-on treatments which come in a can. These, though, have distinct difficulties. Their performance in a fire situation depends entirely on whether the builder applying the brush-on coating covered the timber with a sufficient depth of coating. TTF and WPA both strongly recommend avoiding these brush-on coatings.

A timber product’s ‘reaction to fire’ can be measured, tested and enhanced by the application of a WPA-approved flame retardant treatment.  Timber and panels that can receive such treatments are available in a range of product categories from carcassing to cladding. The WPA also operates respected quality assurance schemes such as its ‘Benchmark FR’ programme for timber treated with a WPA-approved FR product, and ‘FR Build’, which specifically covers FR treated components for the timber frame sector.

What to look for

Reaction to Fire performance is set down legally in a number of European Standard ‘Euroclasses’. Euroclass A represents a non-combustible material, so Euroclass B is the highest level of performance that wood-based products can achieve with suitable flame retardant treatment.

If you’ve been given a spec by an architect, you should stick to it, rather than taking on the burden of responsibility yourself by ordering a different or a cheaper product. You can check that the product matches the architect’s spec by looking at the Declaration of Performance (DoP). This should have been drawn up after the product received its flame-retardant treatment, under CE-marking regulations. Also ask to see the product’s Reaction to Fire Classification Report, issued by an independent third-party certifying organisation. Sellers have a legal obligation to provide DoPs and Fire Classification Reports and buyers have a legal right to see them.

Flame-retardant treatments are very specific, and it’s important to remember this when checking any documentation. If a Classification Report states that Euroclass B has been achieved on 25mm thick Spruce boards, tested without an air gap behind them and on plasterboard, then the fire performance test results are only applicable to exactly those products, installation circumstances, timber species, and board thickness. If no Classification Report is available for the species and design required then it’s not in your interests to proceed: you should find an alternative supplier.

Along with a Euroclass, you may see other wording on a flame-retardant treated wood product. The little letter ‘s’ stands for its smoke generation rating, and s-0 indicates the best performance. Similarly a little letter ‘d’ indicates its rating in generating flaming droplets, which could spread fire: again zero (d-0) indicates best performance. You may therefore see ‘Euroclass B, s-0, d-0’ on a product’s Classification Report or DoP.

What do Building Regs say?

At the end of 2018, the Government revised the Building Regulation in the wake of the Grenfell disaster. Timber cladding is allowed on buildings up to a height of 18 metres. The Building Regulations also state that any material applied to the surface of a building should ‘…adequately resist the spread of flame’.

Free training available

It’s not the builder’s job to specify whether or not a particular situation will require a flame-retardant treatment for timber. Yet it’s certainly worth your while becoming more familiar with the topic so you can raise any appropriate concerns with the architect, homeowner or specifier. Free training on fire-rated timber products from the TTF and WPA is available online at: . You can also freely download Fire Protection Factsheets 1-6 from the Publications section on




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