In May 2014 BSI published a new version of the wood preservation standard BS 8417:2011 + A1: 2014. The main change in this edition was the strengthening of the 30 years desired service life (dsl) specification for use class 4 (ground contact) timber treated with BS EN 599 type preservatives.

The move followed collaborative work by the WPA technical committee, the major preservative manufacturers and its members producing wood for highways contracts. This work highlighted that the performance of longer service life sawn posts may potentially be compromised by exposed heartwood surfaces. Such surfaces are resistant to preservative penetration, which might leave a sawn post under-protected when a 30-year dsl is required.

Variability in the natural durability of heartwood was also a significant factor in strengthening the specification.

It was a growing awareness that the natural durability of heartwood was variable, particularly in plantation grown species that confirmed the justification for an upgrade in penetration requirements.

A step change in penetration requirements

It is important to note that the new standard does not require an increase in preservative retention over what was previously required for 30-year dsl. To ensure adequate protection the penetration class has been increased by one step from NP5 to NP6 for 30-year use class 4 specifications for sawn wood with exposed heartwood faces. This means that for species with sapwood classed as "permeable" (among the softwoods this applies only to pine) full sapwood penetration and 6mm penetration of any exposed heartwood must be achieved. In species with sapwood resistant to impregnation, 12mm sapwood penetration and 6mm penetration of exposed heartwood is required.

Ever since the WPA first alerted treaters to BSI’s plans for the 30-year specification in 2012 we have been at pains to make clear that the focus was on achieving the new penetration requirements.

The WPA is keen to emphasise that any process by which this is achieved is acceptable.

Incising is just one technique by which the depth of penetration in heartwood can be achieved, other techniques can and are being used – the perception that mechanical incising is the only way to compliance is incorrect.

This is also true for round posts pressure treated for a 30-year dsl. No heartwood is exposed in round posts so the sapwood penetration requirement for the species being treated is the relevant factor – if full sapwood penetration has been achieved then the post is BS8417 compliant.

Correcting the misperception

Some fencing contractors believe that only posts that have been incised will satisfy the Highways Agency contract manual and WPA continues to take action to try to set the record straight.

One possible source of the misperception is the National Highways Sector Scheme 2a Fencing document published by the UK Accreditation Service (UKAS) in November 2014.

This included a ‘note’ for information stating that contractors should expect to see incised wood supplied to meet the new 30-year specification. A picture of an incised post is also included in the UKAS note to illustrate the point.

Earlier this year WPA set about trying to get this note corrected. An alternative form of words focused on penetration rather than incising was presented to the SS2a advisory committee and accepted. WPA is now lobbying hard for UKAS to amend and republish the note for information, which should be a major help to correct the misperception about compliance with national highways contracts.

To back this up, the WPA is seeking to extend its third-party quality assurance scheme, the WPA Benchmark to include products treated in accordance with the 30 years dsl.

For use class 4 applications, the WPA Benchmark of quality has to date applied only to products meeting the 15 years desired service life.

However, we expect the first 30 year certificates to be issued in 2016 and this will be a major endorsement of and benefit for sector scheme 4 suppliers.

WPA also feels that more could be done to clarify interpretation of the updated BS8417 30 years dsl treatment requirements and has written formally to the BSI committee responsible for BS8417 to request assistance as a matter of urgency.