The specification requirements for pressure treated softwood deck substructures are set to change in BS8417, the British Standard for wood preservation, when it is next revised later this year, with key trade bodies incorporating the changes now.

This means that all softwood components used for the structural sub-frame of a deck will have to be treated to the same level as components in permanent contact with the ground – Use Class 4.

Pressure treated softwood is commonly used to build decking substructures to support timber and composite deck boards.

Preservative treatment provides wood with added durability, however, it’s a mistake to assume that all pressure treated wood is the same. Whilst one piece of treated wood may look very much like any other, the level of preservative protection could be very different. That’s because BS8417 requires that the loading and penetration of preservative, impregnated into the wood, is tailored to the desired end use.

A review by the Timber Decking & Cladding Association (TDCA) indicates that the most common occurrence of deck failure is with the substructure – beams, wall plates and joists.

Despite clear guidance from organisations like the TDCA, mistakes are still being made in constructing timber deck substructures – which compromise both the performance of the finished deck and the reputation of wood.

This is made worse because once a deck is completed the substructure is typically not visible. The consequences of specification error or bad workmanship during installation does not become evident until it’s too late. For example, if vegetation and/or debris is allowed to accumulate beneath a deck, this restricts air movement and traps water, increasing the exposure risk. Likewise, it is common for deck joists to be set directly on or very close to the ground.

The failure of the substructure is clearly safety critical. In Sweden, Finland, France and north America, the whole deck substructure system (posts, beams and joists) is already categorised as Use Class 4.

Subsequently the decision has been taken to follow this best practice in order to ensure substructures are robust and perform better. Going forward this means that in the UK the whole substructure will be categorised as Use Class 4, whereas previously only the posts were categorised as Use Class 4.

Following a full review of the issues, the Wood Protection Association (WPA) Technical Committee, which includes representatives of the BRE and TRADA, adopted these changes as WPA policy and were subsequently ratified by both the WPA and TDCA boards.

These changes have also been adopted by the BSI committee managing BS8417 (B/515) and so will be incorporated into the full standard review (probably later in 2020). In due course the WPA, TDCA and Timber Trade Federation (TTF) will be working to adopt this change in policy across the membership of all three partner associations, so as to help them transition to the new requirements in time for the publication of the revised BS8417 towards the end of 2020.

In fact TDCA members Hoppings Softwood Products and BSW Timber have already introduced products that meet the new guidance.

This tightening up of the specification for substructures should raise confidence in the market and help to maintain timber’s position as the most popular and cost-effective solution when building decks.