Above:Many above ground fencing components are made from wood species known to be resistant to preservative penetration. BS 8417 now bases the penetration requirement on the thickness of the wood being treated PHOTO: WOLMAN
achieve these requirements but points readers to the WPA manual which does. “This is why compliance with the WPA manual forms the basis for the independent assessment of treated wood quality under the WPA Benchmark scheme,” said Ms Sycamore. There is no one size fits all preservative specification for wood. BS 8417 requires that treatment is tailored to the wood species, the end-use application and the risk posed by decay over the service life desired (15 or 30 years are the desired service life options in BS 8417 for outdoor wood products). The minimum level of preservative penetration and retention is determined by the British Standard Use Class into which end-uses are grouped. Use Class 3 (UC3) is the specification for wood used outdoors not in contact with the ground; Use Class 4 (UC4) is for wood used in direct contact with the ground or freshwater where higher levels of preservative protection are essential.
TIMBER DECK SUBSTRUCTURES The durability of timber used for deck substructures is vital to the structural integrity and long-term performance of the entire installation. Any potential for the early onset of decay would compromise the safe use of the deck and be difficult, costly and inconvenient for the deck owner to repair. “Experience shows that the risk of decay
Penetration classes NP1 and NP2 Penetration class
NP1
is increased when mistakes or short cuts are taken in the design and build phases,” said Ms Sycamore, who cites such instances as not providing for ventilation of the subframe; not clearing the site of vegetation on low level decks; using an untreated or incorrectly treated component in an outdoor environment; and not resealing cross-cuts, notches or holes made on-site with end-grain preservative. Consequently, BSI has upgraded the treatment of softwood deck beams, wall plates and joists from UC3 to UC4 levels of protection – the same as deck and fence posts. To emphasise the point, UC4 treatment is now described as being for “ground contact and external structural support”.
ABOVE GROUND FENCING COMPONENTS
Railing, boarding, arris, featheredge and cant rails not in soil contact continue to be classified in BS 8417 as Use Class 3 components. However, for wood species classified as resistant to preservative penetration such as spruce, and where the default 15-year desired service life is the target, the penetration requirement is now determined by the thickness of the component.
If the component is less than 44mm thick, the penetration class NP1 can be used as an
alternative to the default NP2 penetration detailed in the Standard so long as the preservative retention is not compromised (see table).
This revised specification is based on industry experience with thinner (<44mm) components made from resistant species used in well-ventilated, water-shedding service environments. In addition, resistant species should be treated in a penetrating process that includes a high-pressure phase of at least 60 minutes to ensure the maximum penetration of preservative is achieved.
“This requirement alone should enhance the treatment quality and reliability of products made from resistant species like spruce,” said the WPA.
VERIFYING TREATMENT QUALITY If you’re thinking all this sounds complicated, you would not be alone – the science behind industrial wood preservation standards is extremely detailed. However, all a buyer of treated wood needs to know is that the components they purchase are fit for purpose; responsibility for ensuring compliance with BS 8417 rests with the wood treater. “And the only credible way for a buyer to verify the quality of a treated wood product is if that product has been produced under a third-party verification scheme like WPA Benchmark,” said Ms Sycamore. ■
BS8417: 15 August 2024 Use Class 3 - Penetration requirement for resistant wood
No penetration requirement but the target retention of wood preservative must be present in the outer 3mm analytical zone. NP2
3mm lateral penetration into the sapwood and the target retention of wood preservative must be achieved in the outer 3mm analytical zone. If it isn’t possible to distinguish heartwood from sapwood, the whole sample should be regarded as sapwood.
www.ttjonline.com | November/December 2024 | TTJ
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