27 January 2020

Specification increase for timber deck substructure components

TRADA image

Photo: TDCA.


A common misconception is that all pressure treated wood is the same, but the appearance of treated wood doesn’t correlate to its level of preservative protection.


BS 8417 – the British Standard for wood preservation – requires that the loading and penetration of preservative impregnated into the wood is tailored to the desired end use and service life. There are five Use Classes for wood. The main two for external timbers are:


  • Use Class 3, suitable for above ground exterior use; and
  • Use Class 4, suitable for ground or fresh water contact or exterior structural support.


The most common occurrence of deck failure in both timber decking and composite boarding projects is with the substructure, which is clearly safety critical, difficult or costly to repair, and could – at an extended rate – become a threat to the timber decking market in the UK.


This is despite guidance concerning the correct installation and maintenance of decks, which recommends that any timber substructure is clear of the ground and not in direct contact with vegetation and debris which might accumulate beneath a deck over time, restricting air movement and trapping water. It is also common for deck joists to be set directly on or very close to the ground, and this greater risk of wetting could be considered as being in Use Class 4 even if not in direct contact with the ground.


As a result, other countries – including Sweden, Finland, France and North America – have already categorised the complete deck substructure system (posts, beams and joists) as Use Class 4, where before only the posts fell beneath this Use Class. This increase allows for more robust deck structures and protects the reputation of timber decking against competing materials such as aluminium deck joist systems.


The Timber Decking and Cladding Association (TDCA) and the Wood Protection Association (WPA) agree that the UK should follow suit in order to maintain high standards of best practice, which is in line with recommendations made in TRADA publication Timber decking: The professionals’ manual 3rd edition in 2018.


This increase of Use Class has now been adopted as WPA policy, ratified by both the WPA and TDCA boards, and adopted by BSI committee B515 managing BS 8417 to be incorporated into the full standard review.


TRADA member Hoppings Softwoods has already launched Use Class 4 treated joists.


Further reading

Read Timber decking: The professionals’ manual 3rd edition online with TRADA membership

Buy Timber decking: The professionals’ manual at the Bookshop

Timber Decking and Cladding Association