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Use Class Classification for Deck Substructures

18 December 2019​


Feedback from the Timber Decking and Cladding Association (TDCA) indicates that the most common occurrence of deck failure is with the substructure. This issue affects not only timber decking projects – wood substructures are often used with composite boarding too.


Despite clear guidance being readily available concerning the correct installation and maintenance of decks, the reality is that shortcuts are sometimes taken on the substructure (which is not usually visible post-installation). Property owners often make little attempt to clear vegetation and/or debris which accumulates beneath a deck, restricting air movement and trapping water. In addition, 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. There has been concern that such failures are a growing threat to the future of the timber decking market in the UK. In Sweden, Finland, France and North America, the whole deck substructure system (posts and joists) is categorised as Use Class 4.


Subsequently the WPA and TDCA believe that the UK should follow this best practice in order to help protect the reputation of timber decking and substructures against competing materials.


Following a full review of these issues outlined, the Wood Protection Association Technical Committee (which includes representatives of the Building Research Establishment, 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 WPA and TDCA will be working with the Timber Trade Federation 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).

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