Is your timber specification fit for purpose?

Janet Sycamore of the Timber Decking and Cladding Association discusses why and how the details of timber protection needs to be embraced to ensure specifications are always fit for purpose

Knowledge of fire protection terminology in the design and build sector is woefully low. That’s according to a recent survey of architects by the Wood Protection Association (WPA). The findings of this survey came as no surprise to the Timber Decking and Cladding Association (TDCA), which has long held the view that poor specification of preservative and fire protection treatments leads to compromised timber performance. The TDCA and WPA exist to support architects and specifiers in their correct choice of timber. This specialist knowledge and expertise must be embraced throughout the industry to ensure specifications are always fit for purpose.

The extent of natural durability

How to correctly specify timber for durability is a common question asked of the TDCA. Some timbers (mainly hardwoods) offer natural durability. They can withstand outdoor and ground contact for a determined service life without coating or treatment. BS EN 350: 2016 provides durability classifications for different timber species. There are five classes, Class 1 being the most durable. A table detailing the expected service life of different species for various locations can be found in BS8417 (the British standard for wood preservation).

When to specify preservative treated timber

If you’re intending to use a softwood timber outdoors, some level of factory preservative treatment will most likely be necessary. For outdoor use, timber is generally treated to Use Class 3 or 4 dependant on application, be it above or in ground contact. A proven method for decades, pressure treatment involves impregnating the decking or cladding timber with a wood preservative (most commonly, copper-based). It’s a tightly controlled procedure with checks for the level of treatment and how far the preservative penetrates. BS8417 details the specific requirements for timber preservation. Not only will the right specification of treatment achieve the desired service life, it will also lock up carbon for longer and enable less durable timbers to be effectively used for longer-term applications; an environmental boon. Modified woods are a more recent innovation gathering pace. They employ processes that physically alter the timber to improve durability and many other characteristics. The result is a much wider choice of materials for specifiers to consider. Documented evidence is available from all accredited timber treatment providers. Architects must ask and retain this proof to demonstrate the product will meet the anticipated service life.

Getting to grips with improved fire performance

A risk assessment or Building Regulations usually prompts the need for flame retardant treated timber. Fire (or flame) retardant treatments work by making the wood more difficult to ignite and by slowing the spread of flame, smoke and burning droplet generation, giving more time for evacuation and lessening damage to the structure. Wood and wood-based panels are generally treated to Euroclass C (limited lateral spread of flame, like a BS476 class 1 rating) or Euroclass B (very limited lateral spread of flame, like BS476 class 0). Treatment involves the impregnation of timber under controlled conditions. The smoke rating will be given as an ‘s’ value, and the burning droplets rating as a ‘d’ value. Classification Reports exist to verify performance. They will detail timber species, thickness, substrate e.g. plasterboard, and whether the tested assembly incorporated an air gap or not. Deviation from any of these test specifics – e.g. thickness of the wood – will make the performance certificate invalid. When specifying flame retardant timber, always check the material description in the Classification Report against the material to be used in your project. Species, size and intended use should match exactly. Only then do you have the right evidence to confirm your product’s ability to perform and to comply with the requirements of Building Control officers or insurers.

CE marking and Declaration of Performance

Timber cladding and wood-based panels are subject to CE marking. CE marking compliance is the responsibility of the organisation that brings cladding and panels into the marketplace, and means the product must be accompanied by a Declaration of Performance (DoP). Usually drawn up after treatment of a wood product against fire, insist on a check to ensure your specification matches the DoP. If you need more detail, ask for the product’s Reaction to Fire Classification Report. This is issued by an independent fire test certifying organisation such as Exova.

Where to seek help

Timber treatment is a complex and essential aspect of specification. By knowing what you’re looking for, and appreciating what documentation should be available, you’ll ensure effective specification every time. The WPA has a series of fact sheets to help you, in addition to a CPD accredited course. Telephone helpdesks are available at both the TDCA and WPA. When it comes to timber protection, there’s no reason to make poor specification decisions.

Janet Sycamore is the director of operations at the Timber Decking and Cladding Association