74 | Feature: Treated Wood Performance
SUMMARY
■The WPA/BRE field trial has run for seven years
■There are two UK test sites ■Spruce, larch, Douglas fir and pine are being tested
■The next inspection will be in three years’ time
MAGNIFICENT SEVEN An independent scientific study carried out by WPA and BRE confirms treated wood performance
Above left: The WPA field trial site Birnie Wood in Scotland Above right: The seven year inspection of an incised spruce post at the BRE Garston site
In 2015 the Wood Protection Association (WPA) in collaboration with the Building Research Establishment (BRE), commissioned a long-term controlled field trial of sawn and pressure treated British softwood. The purpose was to test the performance of ground contact fencing timbers in a robust and independent study – the results of which would help to:
• Gain valuable input into the future development of industry technical
standards
• Support the WPA Benchmark quality approval scheme for treated wood
• Build lasting confidence in preservative treated British softwood species in this
application as a capable, reliable and quality product
Seven years on and the findings are very positive – they are showing that wood, preservative treated correctly for its end use, can be trusted to perform in the field.
TEST CRITERIA AND RESULTS There are 80 untreated posts and 600 treated posts (treated in accordance with WPA
TTJ | July/August 2022 |
www.ttjonline.com
Benchmark quality criteria) each installed at two UK test sites with differing soil conditions. Matched samples of spruce (un- incised and incised), larch, Douglas fir and pine are being tested.
The BRE performance assessment of the timber after seven years has shown slightly different results from each site, but the overall outcomes are similar:
• Significant failure of untreated fence posts at both test sites
• No significant signs of change or deterioration in the inspected treated posts
at either site
• In the case of incised and treated spruce, there were no signs of deterioration at
either site
An Information Sheet giving further details of the year seven inspection of this study can be downloaded from the WPA website.
CONCLUSION SUMMARY
Seven years is a significant period in terms of drawing conclusions from the inspection data. The fact that treated posts are still performing
FURTHER INFORMATION For further information on the innovative
WPA Field Trial or any other matter relating to industrial wood treatment, please go to
https://www.thewpa.org.uk or email us via
contact@thewpa.org.uk
well after seven years’ service in two very different sets of ground conditions clearly contradicts the anecdotal ‘conclusions’ being drawn by some in the fencing sector – that following the widespread switch from CCA to copper organic preservatives 12-14 years ago, early failures of posts after three to four years were due to the inferior performance of the new preservatives. Clearly, there must have been other factors involved.
All posts were reinstalled into their holes and the field trial continues until the next inspection. This will be in three years’ time in March 2025, after 10 years in ground contact – these results will be of further interest to all in the timber industry. ■
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