On August 15 the BSI published a new edition of BS 8417. This is the British Standard Code of Practice that sets out the level of preservative treatment necessary for wood to perform effectively in any end-use situation.
The 2024 edition brings several new or revised treatment specifications into effect. For the manufacturers, sellers and buyers of treated wood for outdoor applications there are two revised specifications that need to be complied with – one is for external structural support applications such as timber deck substructures, the other relates to out of ground fencing timbers. This article explains these two changes.
TREATING WOOD CORRECTLY
These revised treatment standards have been anticipated and trialled by the Wood Protection Association (WPA) for some time. “Although BSI took a little longer than anticipated to publish an updated BS 8417, all of the changes made to this British Standard have already been incorporated into the January 2024 edition of the WPA Code of Practice: Industrial Wood Preservation Specification & Practice [WPA manual],” said Janet Sycamore, WPA director.
“This flagship WPA manual is the go-to reference for any business or individual interested in supplying or buying wood that’s correctly treated for its end-use.”
Whilst BS 8417 sets out the minimum wood preservative penetration and retention required to protect wood in any given enduse, it doesn’t provide guidance on how to 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 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.
