Largest ever UK field trial shows treated wood significantly outperforms untreated
8 July 2025
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Interim results of the largest independent assessment of British treated softwood ever carried out in the UK demonstrate that treated wood for ground contact applications such as fencing can be relied on to provide a long service life.
The 10-year UK field trial of hundreds of wooden fence posts and test stakes was commissioned by the Wood Protection Association (WPA) and carried out by the Building Research Establishment (BRE) at two test sites: at BRE Garston, Watford, and Birnie Wood, Elgin, in the north of Scotland. It shows a substantial improvement in durability and resistance to decay when compared to untreated timber products.
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The majority of the untreated wood posts at the sites have now completely failed due to fungal decay, with data suggesting an average service life of less than five years. The study also notably shows that posts made from larch are failing faster than any other species.
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“These results clearly show that untreated wood of any kind is not fit for purpose for ground contact applications,” said WPA Director Neil Ryan. “Only wood that has been correctly treated gives the performance required to ensure long-term success.
“The study also dispels the myth that larch is durable and can be used in this way without a preservative treatment.”
After 10 years, the preservative-treated posts in the trial are performing well, particularly incised spruce. Some opening of incisions and elevated moisture content has been noted, but without deterioration of the wood.
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Across all species and treatment types in the trial, there have been only isolated failures of treated posts. “This is not entirely unexpected after 10 years. These are clearly outliers on the bell curve of preservative performance, with others in the same species and preservative group rated zero – meaning they still have no visible decay present at all,” said Ryan.
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WPA Chairman Steve Young said: “This landmark independent study demonstrates clearly that preservative treated wood works outdoors. It confirms that correctly treated wood is the only reliable choice for ground contact applications.
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“At a time when businesses’ sustainability obligations are increasing, correct treatment allows them to use wood – the only endlessly renewable construction material – for the maximum possible time.”
The WPA set up the field trial in 2015 in partnership with BRE and a group of industry sponsors, with the goal of helping develop industry standards, including BS 8417 and the WPA Code of Practice – to support WPA Benchmark quality approval schemes for treated wood, and to build market confidence in preservative-treated British softwood species.
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After the completion of the initial 10-year term, the project has been extended for a further five years.​
ABOUT WPA
The Wood Protection Association acts as a technical and advisory body providing guidance to the timber products supply chain, specifiers, contractors, engineers and architects on the best ways to ensure wood is fit for its intended purpose.
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By actively promoting and developing the technology that enhances the performance and value of sustainable softwood products – mainly through preservation, wood modification or flame-retardant treatment – the WPA also enables the timber supply sector and project designers to make the most of wood and realise its full potential.
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WPA is the UK lead body on all matters concerning wood protection and has a respected voice with many British and European Standards setters. In addition, we have strategic partnerships in place with other major trade associations, so that we speak with one voice on such issues.