Why the BS8417 30 yrs spec. has changed

In May 2014, BSI published an updated version of the wood preservation code of practice -  BS 8417: 2011 + A1: 2014.  The main change in this edition of was to the 30 year desired service life specification for use class 4 (in ground contact) commodities. The change affects those specifying or buying treating wood for highways contracts and other safety-critical or high quality fencing applications. 

Why the changes?

Industry was increasingly aware that the natural durability of plantation grown commercial wood species was more variable than previously assumed. British standard classification of natural durability is based on the heartwood of a species and, as a consequence of the increasing variability, the performance of longer service life sawn posts may be compromised by exposed heartwood surfaces. Such surfaces are resistant to preservative penetration and hence may be under-treated. Fence posts sawn from a species like Douglas fir, for example, are likely to have a high proportion of heartwood on exposed faces. The 30 yr desired service life specification in BS8417:2014 was upgraded to requires exposed heartwood faces to be penetrated with preservative to 6 mm in addition to sapwood penetration requirements.

Accounting for substrate variability

It is known widely that penetration levels into a naturally variable material like wood will inevitably also be variable - even when using the same species and preservative impregnation process. Therefore an Acceptable Quality Level (AQL) is applied to penetration requirements within BS 8417. Typically for resistant species the AQL is set at 25%. In addition, even within the 75% of samples which must pass, the enhanced heartwood penetration required for a 30 year desired service life is only required to be visible in 75% of the exposed surface area of that heartwood.

Achieving the new requirements

Any process that results in the achievement of the 6mm heartwood penetration requirement may be used. Mechanical incising is mentioned in the standard as one example of a technique which has been used in the past to help enhance preservative penetration in resistant species. Unfortunately, some specifiers and contractors have interpreted this wording as requiring that incising must be used in order to satisfy the new 30 year specification. This is not correct - penetration is the key factor and any method of achieving the requirements of this revised standard is the responsibility of the timber treater. WPA is endeavoring to raise awareness on this point within the supply chain.

Posted at 6:20 pm on October 29th, 2015