Feature: Wood Preservation | 59
“Fully transparent performance data from a real-time scientific study like this is vital to building trust in preservative treated softwoods,” he said.
ABOUT THE FIELD TRIAL
A total of 1,200 fence posts, 160 untreated posts and 300 EN252 site control reference stakes were installed across two field test sites – BRE Watford with London clay loam and Birnie Wood, Elgin, Scotland with sandy loam overlying gravel deposits. Under the terms of the field trial contract, BRE was required to verify that the production of the 75mm x 75mm sawn posts was consistent and in line with commercial practice.
This involved audits and observation visits to assess the incising, the three different copper organic preservatives being used and to ensure that the impregnation process complied with BS 8417 for ground contact (Use Class 4) applications. “We knew that all eyes would be on the data generated, so we have been meticulously thorough in documenting our procedures and findings at every stage,” said BRE’s Ed Suttie.
YEAR 10 REPORT DELIVERS TANGIBLE PERFORMANCE DATA Earlier last year, BRE carried out the 10- year inspection of both sites and reported the findings to WPA who then briefed the sponsors. ‘
“This is the most significant report to date as it gives measurable insights into the performance of different species and the benefits that accrue when wood is correctly prepared and impregnated for ground contact applications,” said WPA field trial consultant, Neil Ryan.
That’s because most untreated wood controls at both sites have now failed completely due to fungal decay, with the BRE data indicating a service life of less than five years.
“This demonstrates clearly that softwood species with low natural durability should never be used for ground contact applications unless properly treated,” said Mr Ryan. Another notable revelation of the BRE report is that untreated larch posts are failing faster than any other species.
“WPA has had its suspicions about the natural durability of larch for some time,” said Mr Ryan, who emphasised that the field trial data clearly contradicts and dispels the widely held perception that larch is naturally durable and can be used without preservative treatment for ground contact applications.
BRE reported that all treated pine, Douglas fir and spruce posts continue to perform well. It is notable after 10 years in the ground that the incised and treated spruce posts are also standing up well. BRE stated that it is seeing some opening of the incisions and an elevated moisture content in these posts at ground line, but this is not resulting in a structural deterioration of the wood.
There are isolated failures of treated posts across all species and preservative types
which BRE pointed out was not entirely unexpected after 10 years. The report pointed out that these are clearly outliers on the bell-curve chart of preservative performance and all other posts in the same species and preservative group are not displaying any visible signs of decay.
“What is encouraging for the WPA and other sponsors is the irrefutable evidence that correctly treated UK softwoods can perform reliably in ground contact applications,” said Steve Young. “We are eternally grateful to the sponsors who have backed this important project and the decision to extend the field trial to 15 years – the next five years will almost certainly produce more extensive performance data to help fulfil the potential of home- grown softwoods.” ■
Above: WPA Benchmark is a quality assurance scheme that independently verifies a wood product is correctly treated for its application and compliant with UK standards
Organisations that made the field trial possible Project lead Wood Protection Association Independent project management: Building Research Establishment Project sponsors: Wood Protection Association; Scottish Forestry; Scottish Enterprise; Grown in Britain; James Jones; BSW Timber; Arxada; Wolman; Koppers; Confor; Timber Development UK; Timber Decking & Cladding Association; Wood Campus; Bond Timber; James Davies; Munro Sawmills; M&M Timber; Calders & Grandidge; CJ Timber
www.ttjonline.com | Spring 2026 | TTJ
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