Recently the National House-Building Council appealed to the Wood Protection Association (WPA) for assistance. It wanted continuing professional development training to help its London building control surveyors get their heads round timber fire retardant use and specification.

The episode told a story. Timber is being used increasingly overtly, in an ever wider range of applications in UK construction. Besides growth in timber frame, there’s been increasing use of other timber-based structural systems, plus a surge in the popularity of wood cladding and shingles. These ‘new’ applications have raised new questions about the technical performance of timber products in them and prominent among these have been queries about their fire performance.

Adding interest and complexity to the subject are latest developments of fire safety legislation and standards.

“Dynamic transition best describes what is happening in the fire protection industry,” added Arch fire retardant sales support manager Jacqui Clay. “The introduction of new European reaction to fire standards is changing the way we think about specifying fire protected timber.”

The big news on this front is the European reaction to fire standard prEN15912. This is at consultation stage and some way from enactment, but promises important developments in timber fire protection emphasis and the treatment sector is being urged to adapt to it now.

“The WPA is already amending and strengthening criteria for product assessments based on the draft,” said Andrew Hughes, technical director of Arch Timber Protection and chair of the WPA technical committee.

Among prEN15912’s stipulations is that durability of fire retardant protection is considered. “It clearly makes sense for the service life of the retardant to match the product’s,” said WPA director Steve Young.

The new standard also evaluates impact of treatments on moisture absorption. “This is also important as some can cause staining, decay, poor paint adhesion, metal corrosion, and migration of chemicals in high humidity,” said Young. “That’s why WPA already classifies treatments as DI, for dry interiors, HR, for humidity resistance, and LR, for high humidity application.”

Another change under way is the transition from the old British Standards system of timber product fire classification to the European. This forms part of the harmonisation of EU building sector standards under the Construction Products Directive (CPD). Under the new approach, fire performance for timber used in “permanent constructions” is classified Euroclass B or C; the former being for products in key structural features, staircases and other designated escape routes, the latter for areas where lower ratings are acceptable, like buildings with no common access areas and shorter designated escapes. Whether they meet the classifications depends on performance in the ENISO11925 ignitability and EN13823 single burning item tests, and these must be done by notified bodies approved to issue European Classification reports.

The classification process also takes into account timber species, thickness and density and “field of application”, which means whether it’s mounted with or without an air gap behind and has a backing material.

CE marking of fire retardant-treated timber is also coming into the equation where there is an EN product standard for a type of retardant-treated product (which currently means solid wood panelling and cladding and wood-based panels in construction). In the UK, CE marking isn’t yet compulsory, but that could change in the next few years.

Against this backdrop, it’s not surprising timber users, specifiers and merchants are approaching timber treaters for fire retardant guidance. “The growth in the use of timber frame and such products as cladding means that there are people coming to this issue for the first time, and there’s demand for education,” said Neil Ryan, managing director of PTG Treatments, one of the UK’s biggest industrial timber pre-treaters. “We spend quite a bit of time giving advice. Just recently we’ve been called increasingly by timber framers, but it’s also local authorities, merchants, building inspectors.”

At the Kielder observatory in Northumberland the timber was treated with Arch’s Non-Com Exterior surface spread of flame protection while plywood used in the interior was supplied with Dricon treatment

When fellow-WPA member Arch is quizzed on the topic its advice is to ask three key questions about fire retardant-treated products. First, whether the treatment complies with prEN15912. Second is if the treated product is independently certified with an EU Classification Report issued by a notified body for the specific timber and application. Last but not least, it urges specifiers and end users to ensure fire retardant treatments have been made and applied “correctly, under controlled conditions with full traceability”.

“Fire retardant performance is best assured if manufacturing and application is carried out under an independent certification and accreditation scheme,” said Clay. “These include ISO 9001, British Board of Agrément accreditation and WPA listing and approved status.”

The WPA’s Flame Retardant Quality Scheme, which lists companies authorised to apply flame retardant treatment of timber and wood-based materials by pressure impregnation, has also been developed for use as a “BS EN ISO 9001 quality schedule”. And these treaters are included, along with listed retardant products, in the organisation’s Fire Retardant Specification Manual.

“The ultimate aim of fire retardants is to protect lives and the onus is on specifiers to ensure compliance [with latest standards],” said Clay. “So you should choose both product and service provider carefully.”

Another view from the representatives of the treatment sector we spoke to is that it is generally safer to opt for impregnation pre-treated products rather than site-applied surface fire retardant treatments. “When they’re applied by brush or spray on site, fire performance can only be assured if application is covered by an independent installer accreditation scheme,” said Clay.

“There are particular question marks over the durability of site applied treatment and its ability to meet prEN15912,” said Young. “But a WPA test programme has shown that, after 21 years, industrial impregnation pre-treated products suffer no loss of performance .”

The WPA provides further guidance on selecting fire retardants in its specifiers’ checklist (summarised left). This will also form part of its updated Fire Retardant Manual, due out this year.

“In addition, we’re developing our online and face to face CPD training,” he said. “ The NHBC surveyors take their CPD course in January.”

The WPA Fire Retardant Specifiers’ Checklist in summary

1. Before specifying a fire retardant treatment for a specific component, check: a) the fire performance required – is it Euroclass B or C, or BS 476 Part 6 or 7 (Class O and Class 1); b) the service environment – is it internal dry, internal humid or external?
2. Verify product performance credentials and ensure they’re corroborated.
3. If performance to a European Standard is required (Euroclass B or C), ensure the product is backed by a Classification Report from a Notified Body.
4. Ensure the product description in the Classification Report is applicable to the timber species in the project.
5. Select a fire retardant with an Extended Application Report, defining
application scope.
6. For external applications of fire retardants insist on evidence of adequate performance after weathering. 
7. Ensure the manufacture of flame retardant products and their application to timber is covered by a third-party quality assurance scheme.
8. If brush or spray site-applied surface coating fire retardants are specified, ensure application is carried out under independent certification and accreditation schemes for installers.
9. If in any doubt and for this checklist in full contact the WPA www.wood-protection.org.

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