Raising awareness of treated timber

The Wood Protection Association (WPA) and its members champion the use of building with timber – developing and promoting technologies that enhance its capabilities and value as a building resource.

If you work with wood, you’ll know your choice of timber is vital to the look and cost of a project. However, in terms of longevity and performance, it’s also important to consider specifying an industrial, factory applied pre-treatment.

Flame Retardants (FR) for timber generally work by reducing the surface spread of flame, heat and smoke release – allowing more time for escape. Heat or chemical wood modification processes positively change the characteristics of timber, eg, by creating improved durability and dimensional stability. Timber preservatives for softwood components, applied by pressure impregnation, provide tailored levels of protection from decay and insect attack.

Why and when does timber need protection?

Both softwood and hardwood timbers have natural properties that lend themselves well to use in building applications.

To meet today’s building standards and codes of practice, and to enhance performance in service, timber’s natural ability to perform sometimes needs additional help. These situations can be categorised into two areas:

  1. enhancing timber’s performance in a fire
  2. extending the service life of timber by enhancing its ‘durability’ (resistance to decay and insect attack).

Timber and fire performance

The factory application of a FR treatment uprates timber’s ‘reaction to fire’ classification.

Data obtained from reaction to fire testing results in that material being given a Euroclass performance rating. Untreated wood-based materials typically have Euroclass ratings of D or E. Flame retardant treatment can improve that to Euroclass B or C as required by the specification and end use.

Factory application is carried out prior to installation, ensuring all faces of the timber or board product can be protected. Because of this, the application of flame-retardant products on the construction site, by brush or spray, is not approved by the WPA as quality control is almost impossible to assure.

In the wake of the Grenfell fire tragedy, Building Regulations have changed. Amendments to Approved Document B(1) (July 2022) confirm that, in buildings with a ‘residential’ purpose and a storey of 11m or more in height, elements such as cladding, balconies, and other external surfaces must achieve a Euroclass A1 or A2-s1, d0. While some exemptions apply, this effectively rules out the use of combustible materials for these elements.

While it is not possible to enhance any organic substrate, including wood, to a Euroclass A rating, flame retardant treatments enhance safety, add value and continue to enable timber cladding to be suitable for many low rise, domestic housing or non-residential applications in compliance with Building Regulations.

© Wood Protection Association. Preservative timber treatment in a vacuum pressure impregnation process.

Industry statement

The WPA, in agreement with other trade bodies, strongly advocates that an independent, professional fire risk assessment is carried out at the design stage for multi-occupancy and assembly buildings such as community centres and schools – regardless of height. These should consider building design, use, materials and location.

Furthermore, in such buildings, timber-based cladding and balcony components should be treated using a quality assured factory-applied flame retardant to Euroclass B performance levels – unless shown NOT to be necessary by an appropriate risk assessment process.

A checklist (Guidance Note WPA FR6) summarising the essential factors of an effective and accurate specification is available from the WPA website, along with more information regarding FR treatment plus a list of approved suppliers and accredited products.

Timber durability

Depending on its natural durability and end use, a timber component may need extra protection through preservative treatment or wood modification.

Nowhere is the need to build confidence in the efficacy of treated wood more acute than in the fencing and landscaping sector where products are used in direct contact with or very close to the ground. These are known as Use Class 4 (UC4) applications – where wood is permanently exposed to the weather and wet for extended periods of time. Fifteen years is the default service life for UC4 applications cited in BS8417: Code of Practice for preservative treated wood. This standard also specifies the level of preservative treatment necessary to protect wood in the other applications. Including Use Class 2 for internal construction applications such as roof trusses and floor joists and UC3 for exterior, out of ground applications such as fence rails, decking boards and timber cladding.

Clear messaging

As part of the WPA’s ongoing timber treatment awareness campaign, simplified guidance resources are available which focus on differentiating between interior and exterior applications and the importance of treating ground contact wood using the Make sure it’s 4 branding.

The general consensus is that knowledge of when to specify UC4 levels of treatment has grown significantly in the past two years and there’s more UC4 treated material for sale in the timber supply chain, but there’s still the question of how buyers can be sure they are getting what they ask for.

Finding a trusted treatment supplier

That’s where the WPA Benchmark quality accreditation scheme comes in.
The WPA operates independent quality schemes to underpin confidence in both the efficacy of treatment formulations and their application to the wood substrate.

The WPA Benchmark Quality Approval Schemes cover both FR and preservative formulations, processes and products – providing valid independent assessment and verification. 2022 saw a marked increase in the number of businesses awarded a WPA Benchmark quality accreditation (preservative Treated Wood), especially from overseas companies who supply to the UK market.

Should you be looking for a trusted supplier, all WPA Benchmark accredited businesses are listed on the WPA website.

If you want to demonstrate you are a trusted supplier and become an Approved Treater, get in touch with the WPA to find out more.

www.thewpa.org.uk