Tighten up on deck

When it comes to designing products for durability, the Timber Decking and Cladding Association (TDCA) has encountered a disturbing lack of knowledge, prompting a call for tighter specifications

The Timber Decking and Cladding Association (TDCA) is appealing to specifiers to put it to use when designing products. The organisation is appealing for specifications to be tightened up by detailing the durability required of a product in line with standards, something its online resources can help them achieve. All told, the information it has available ought to make it a first port of call for those designing or installing decking or cladding.

It’s not just about information; the organisation can help in the sourcing of accredited materials, which it says should also be high on the list, if specifiers want to be certain that timber will perform as required. To this end, the TDCA operates the DeckMark and CladMark quality accreditation schemes for its members.

Janet Sycamore, the TDCA’s director of operations explained the organisation’s concerns. “In general the enquiries we receive suggest a distinct lack of knowledge about specifying and designing for durability,” she said. “For example, many (including some within the timber industry) do not appreciate that the sapwood of all timber species is perishable i.e. not durable. This applies equally to hardwood as well as to softwood species.

"Therefore if you are intending to use a naturally durable hardwood species, be aware that the natural durability classification applies only to the heartwood (reference BS 350-1). It’s likely that different grades of timber will be available and the best will limit the amount of sapwood present. “Another common misconception relates to preservative treated softwood (and also hardwood if treated).

"People believe that there is a ‘one size fits all’ solution when there isn’t. The fact is the best way to treat timber is in a factory controlled industrial process where nowadays it is treated according to its end use. Timber destined for ground contact needs more protection than internal building timbers for example. Therefore a roof joist is not suitable as a joist for external decking. Tailoring treatment in this way allows the correct amount of preservative product to be used for the job, precisely the right approach from an environmental perspective, but awareness is needed to avoid mistakes.”

The standards that relate to timber treatment are “abundant, technical and some would say rather convoluted”, she said, but nonetheless there is some fundamental information that will help.

This includes the different end uses in which timber can be placed are grouped into Use Classes (the relevant standard is BS EN 335-1). As far as decking and cladding are concerned anything that is above ground contact is designated as Use Class 3 (external above ground contact) and anything in ground contact is Use Class 4 (external, ground or fresh water contact)

There is then a choice in terms of the Desired Service Life (DSL) to achieve; 15, 30 or 60 years are the options available. With decking 15 years is the industry default position and for cladding it’s generally 30 years. Anything more than this will probably require a specific request to the supplier. Once a decision is made, it should be insisted upon that the supplier provides supporting documentation stating what the product conforms to.

The UK industry standard for timber preservation is BS8417. This ought to be referenced in the paperwork together with the Use Class and the associated Desired Service Life “We are asked about this topic almost daily,” Sycamore added.

“We emphasise that if the supplier is unwilling or unable to confirm any of the above we recommend that you walk away and source your material elsewhere. The reason we are categorical about this is if things go wrong, timber is often automatically blamed which undeservedly affects perceptions and reputation.”

The TDCA offers help to professionals through its series of websites. One is dedicated to cladding, one to decking and another – The Knowledgebank – provides learning resources, member literature and case studies. There’s also a telephone enquiry service to help, if particular requirements for information are not covered on these sites. If a better understanding of timber treatment and the standards relating to it are required, then the Wood Protection Association produces a manual that encompasses all the different standards and requirements. This is a must-have document for anyone involved in specifying or using pressure treated timber on an ongoing basis.