FEATURE DECKING & CLADDING
‘Good to Know’ Knowledge Bytes The Timber Decking & Cladding Association (TDCA) is focusing in 2022 around highlighting the key information that those buying, selling and using timber decking and external timber cladding should be aware of.
A
specific subject is the size of cladding profiles, specifically tongue and groove (T&G) which is an interlocking system where size matters most. According to BS8605 – External Timber Cladding Part
1: Method of Specifying, for medium movement timbers such as Siberian larch, the profile width for a T&G board is restricted to a maximum of 125mm. Further guidance on tongue dimensions and movement gaps is provided, with bigger gaps and tongues being required for wider boards. At 125mm wide the standard requires a minimum movement gap (mg) of
treated to a Use Class 4 level of protection, irrespective of whether the
4mm, tongue width (tw) at least 15mm and thickness (tt) at least 7.5mm.
material is in ground contact or not. This reclassification to Use Class 4 is
General width to thickness ratio for all profiles is between 4:1 and 6:1.
a recent change which the industry is gradually adapting to. Availability
In summary, cladding boards that are essentially designed for internal
of UC4 treated joists is increasing especially through companies who are
use, where the effects of moisture are not so important, should not be
members of the TDCA. An industry campaign highlighting the importance
used externally.
of correctly treated timber is currently being promoted throughout the
T&G boards should not be fitted tightly together even though there
timber supply chain – learn more at the Wood Protection Association
may be a strong temptation to do so. Allowing for the natural movement
website: www.thewpa.org.uk/make-sure-it-s-4
of the timber mitigates problems arising; tongues don’t become
Getting the fundamentals right at the beginning of any decking or
disengaged, and the cladding stays secured to the wall – so long as the
cladding project is key to both customer and seller satisfsaction. Armed
correct fasteners are used, but that is a topic for another day.
with the appropriate products and information helps to ensure problems
Cladding suppliers need to signpost customers to the correct profile
are kept to a minimum.
for the end use and would be wise to comply with the guidance in
For those looking for added assurances of quality, the TDCA operates
BS8605. This standard (which may be updated soon) remains a relevant
DeckMark and CladMark quality accreditation schemes which cover
document and contains guidance on other cladding profile styles including
products, suppliers and installers. They provide audited verification of
rectangular, shiplap, parallelogram and featheredge.
compliance with good manufacturing and supply practices and sound
Regarding decking, our attention is on one of the most common deck
installation techniques trusted by the TDCA and the wider industry.
issues – early joist failure. When selecting materials the emphasis is often on the deck boards but consideration to the deck substructure – posts,
Want to become accredited? Contact the TDA directly at
beams, joists, blocking and bracing – is just as important as it forms the
info@tdca.org.uk. Only quality oriented companies should apply.
foundation to the whole project. Pressure treated softwood is the go-to
For summarised guidance, The Timber Cladding Handbook produced
material for deck substructure components.
in collaboration with the Timber Trade Federation is available for £12 (inc p&p) from the TDCA website www.tdca.org.uk/publications
To ensure it is fit for purpose it must be preservative pressure Timber Construction Magazine
Spring 2022
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