NEW BS 8417 TIMBER TREATMENT STANDARD COMES INTO EFFECT
The 2024 edition brings several new or revised treatment specifications into effect. For manufacturers, sellers, and buyers of treated wood for outdoor applications, there are two revised specifications that need to be complied with:
- External structural support applications (e.g., timber deck substructures)
- Out-of-ground fencing timbers
This article explains these two changes.
Treating Wood Correctly
These revised treatment standards have been anticipated and trialed by the Wood Protection Association (WPA) for some time. WPA Director Janet Sycamore explains:
“Although BSI took longer than expected to publish an updated BS 8417, all the changes have already been incorporated into the January 2024 edition of the WPA Code of Practice: Industrial Wood Preservation Specification & Practice (WPA manual).”
This WPA manual is the go-to reference for anyone interested in supplying or buying wood that is correctly treated for its end use.
While BS 8417 outlines the minimum preservative penetration and retention required for protecting wood in various end-use scenarios, it does not explain how to achieve these standards. Instead, it directs readers to the WPA manual. As Janet notes, “Compliance with the WPA manual forms the basis for independent assessment of treated wood quality under the WPA Benchmark scheme.”
No One-Size-Fits-All Solution
There is no universal preservative specification for wood. BS 8417 mandates that treatment must be tailored to the wood species, end-use application, and the risk of decay over the desired service life (either 15 or 30 years for outdoor wood products). The British Standard Use Class system groups end uses:
- Use Class 3 (UC3): Wood used outdoors, not in contact with the ground
- Use Class 4 (UC4): Wood in direct contact with the ground or freshwater, requiring higher levels of preservative protection
Timber Deck Substructures
The durability of timber deck substructures is critical to the structural integrity and long-term performance of the entire installation. The early onset of decay could compromise safety, making repairs costly and inconvenient. Janet highlights common issues such as improper ventilation of subframes, vegetation not cleared from low-level decks, and using untreated or incorrectly treated components.
To address these risks, BSI has upgraded the treatment of softwood deck beams, wall plates, and joists from UC3 to UC4 protection levels—equivalent to deck and fence posts. UC4 treatment is now specified for “ground contact and external structural support.”
Above Ground Fencing Components
For railing, boarding, arris, feather edge, and cant rails not in soil contact, BS 8417 continues to classify them as Use Class 3 components. However, for wood species resistant to preservative penetration (like spruce), and where a 15-year service life is the target, the penetration requirement is now determined by the thickness of the component.
- Components less than 44 mm thick can now use penetration class NP1 as an alternative to the default NP2, as long as preservative retention is not compromised.
This revised specification is based on industry experience with thinner components made from resistant species used in well-ventilated, water-shedding environments. Additionally, resistant species should undergo a high-pressure phase of at least 60 minutes during the treatment process to maximize preservative penetration.
Verifying Treatment Quality
If this all sounds complicated, you’re not alone. The science behind industrial wood preservation standards is intricate. However, buyers only need to ensure that the wood components they purchase are fit for purpose. Responsibility for compliance with BS 8417 lies with the wood treater.
The only credible way for a buyer to verify the quality of treated wood is through a third-party verification scheme, such as WPA Benchmark.
For more information, visit www.thewpa.org.uk.