FEATURE: CLADDING & DECKING
Deck Substructures: Prepare for New Specification Levels
As the specification requirements for pressure treated softwood deck substructures are set to change, Janet Sycamore at the Timber Decking and Cladding Association (TDCA), explains how this will improve performance for timber decking.
B
S8417, the British Standard for wood preservation, when
it’s too late. For example, if vegetation and/or debris is allowed to
it is next revised later this year will mean that all softwood
accumulate beneath a deck, this restricts air movement and traps
components used for the structural sub-frame of a deck
water, increasing the exposure risk. Likewise, it is common for deck
will have to be treated to the same level as components
joists to be set directly on or very close to the ground.
in permanent contact with the ground, referred to as Use
The failure of the substructure is clearly safety critical. In Sweden,
Class 4. Pressure treated softwood is commonly used to build
Finland, France and north America, the whole deck substructure
decking substructures to support timber and composite deck boards.
system (posts, beams and joists) is categorised as Use Class 4.
Preservative treatment provides wood with added durability and
Subsequently the decision has been taken to follow this best
BS 8417 requires that the loading and penetration of preservative,
practice in order to ensure substructures are robust and perform
impregnated into the wood, is tailored to the desired end use.
better. In addition this should raise confidence in the market and help
End uses are grouped into five categories: Use Class 3 relates to
to maintain timber’s position as the most popular and cost effective
exterior above ground use whereas Use Class 4 relates to exterior
solution when building decks. Going forward this means that in the
ground contact use. Ground contact use presents a higher biological
UK the whole substructure will be categorised as Use Class 4, whereas
risk to timber and therefore, in order to work well in this situation, a
previously, only the posts were categorised as Use Class 4.
higher level of wood preservative needs to be impregnated into the
Following a full review of the issues outlined, the Wood
timber. The ‘Use Class’ system is important because it indicates the
Protection Association (WPA) Technical Committee, which includes
scale of the biological threat faced by a wood product in a specific
representatives of the BRE and TRADA adopted these changes as
end-use situation and determines the preservative loading and
WPA policy and were subsequently ratified by both the WPA and
penetration required to deliver the service life required.
TDCA Boards.
Clear information about using wood correctly for decking and
These changes have also been adopted by the BSI committee
landscaping projects has been promoted widely in the UK for many
managing BS8417 (B/515) and so will be incorporated into the full
years with organisations like the TDCA leading the way in providing
standard review (probably later in 2020). In due course the WPA,
supply chain and end-user good practice guidance.
TDCA and Timber Trade Federation (TTF) will be working to adopt
Despite this, mistakes and shortcuts are still being made in
this change in policy across the membership of all three partner
constructing timber deck substructures – which compromise both the
associations, so as to help them transition to the new requirements
performance of the finished deck and the reputation of wood. This is
in time for the publication of the revised BS8417 towards the end of
made worse as once a deck is completed the substructure is typically
2020. In fact TDCA members Hoppings Softwoods and BSW Timber
not visible with the consequences of bad practice not evident until
have already introduced products that meet the new guidance.
Spring 2020
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