A long-running field trial led by the Wood Protection Association (WPA) is providing some of the most comprehensive real-world performance data yet on the durability of preservative-treated UK softwoods, as the project reaches its ten-year milestone.
Conceived and commissioned by the WPA in 2015, the 15-year field trial was established to evaluate the decay resistance of fence posts manufactured from British-grown pine, spruce, larch and Douglas fir. The trial – the largest of its kind undertaken in the UK – is managed independently by the Building Research Establishment (BRE) and is backed by a coalition of forestry, sawmilling and wood preservation organisations from across the supply chain.
According to the WPA, fencing remains one of the most important end markets for preservative-treated softwoods, but long-term, independent evidence has historically been limited. “Fencing is an important market for treated softwoods and a long-term, independent, field trial is the only credible way to establish if homegrown softwoods can perform reliably when used for fence posts,” said WPA Chairman Steve Young.
The scale and duration of the trial were deliberately designed to reflect commercial reality rather than laboratory conditions. A total of 1,200 fence posts were installed across two contrasting UK sites, alongside 160 untreated control posts and 300 EN252 site control reference stakes. The test locations were BRE Watford, with London clay loam soils, and Birnie Wood near Elgin in Scotland, characterised by sandy loam over gravel deposits.
Alongside monitoring in-service performance, BRE also verified that the fence posts were produced in line with normal commercial practice. Crucially, the trial was designed to reflect real-world commercial conditions, with posts manufactured, treated and installed in line with normal industry practice rather than laboratory-controlled assumptions. This included audits and observation visits to assess incising processes, the use of three different copper organic preservative systems, and compliance with British Standard BS 8417 requirements 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.
Earlier this year, BRE completed the ten-year inspection of both sites and reported its findings to the WPA and trial sponsors. WPA field trial consultant Neil Ryan described the results as the most significant to date, providing measurable insights into both species performance and the impact of correct preparation and treatment for ground contact use.
One of the clearest findings to emerge from the ten-year inspection is the contrast between treated and untreated material. BRE data shows that most untreated control posts at both sites have now failed completely due to fungal decay, with the data indicating a service life of less than five years. According to Ryan, this demonstrates that softwood species with low natural durability should not be used for ground contact applications unless properly treated.
The performance of untreated larch is particularly notable. Trial data shows that untreated larch posts are failing faster than any other species included in the study, contradicting a widely held perception that larch offers sufficient natural durability for ground contact use without preservative treatment. Ryan said the findings clearly dispel this assumption and reinforce the importance of evidence-based specification.
By contrast, BRE reports that all treated pine, Douglas fir and spruce posts continue to perform well after ten years in the ground. This includes incised and treated spruce posts, which have maintained structural integrity despite some opening of incisions and elevated moisture content at ground line. BRE states that these moisture levels have not resulted in structural deterioration of the wood.
For the WPA, the value of the trial extends beyond the immediate findings. Young confirmed that, alongside reporting results for the exclusive use of sponsors, the data collected will also inform any future changes to preservative treatment specifications promoted by the association. These specifications are intended to achieve the 15-year service life for treated wood set out in BS 8417 and are verified through the WPA Benchmark quality assurance scheme.
“In this way the investment by sponsors will have a lasting benefit for the UK forestry and wood treating industries as a whole,” Young said. Industry stakeholders have also highlighted the broader significance of the project for the UK timber supply chain. Confor Chief Executive Stuart Goodall described the strong backing for the trial as “a much-needed collective commitment to advance commercial opportunities for homegrown softwoods,” while Timber Development UK technical manager Nick Boulton emphasised the importance of transparency. “Fully transparent performance data from a real-time scientific study like this is vital to building trust in preservative treated softwoods,” he said.
As the field trial continues toward its full 15-year duration, the WPA expects the data to play an increasingly important role in shaping guidance, standards and market confidence. For an industry seeking to maximize the performance and credibility of UK-grown timber, the findings underline the importance of long-term evidence, correct treatment processes and realistic assumptions about durability in service.
More at www.thewpa.org.uk
