WPA investigates early failure of preservative treated gate posts
When four square sawn preservative treated softwood gate posts, installed in mid-2021, started to decay prematurely, the Association of Fencing Industries (AFI) contractor who installed them asked the Wood Protection Association (WPA) to investigate what went wrong.
These large cross section posts were set 600mm deep and held in place by concrete. They supported field gates and the contractor confirmed that they were intended to achieve the 15-year service life specified in the wood preservation code of practice, and BS8417 for preservative impregnated wood in Use Class 4 (direct ground contact) conditions.
WPA agreed to investigate the posts as part of a joint initiative with the AFI to get to the root cause when treated wood posts purchased and installed by an AFI contractor did not meet performance expectations. Two cross section samples were cut from one of the posts and sent to the WPA for expert analysis and comment. One sample was from the heavily decayed groundline failure zone to allow assessment of decay and one from a sound, above ground, section of the post to enable wood preservative penetration and retention (loading) to be determined.
What the WPA investigation found
Timber species and preservative penetration
The post was visually identified as pine, with sapwood clearly distinguishable from heartwood.
Spraying the crosscut surfaces of the posts with a copper reagent confirmed that the preservative used in the impregnation process had penetrated the sapwood of the pine post, and although the extent of the penetration was a little inconsistent, it was deemed to be satisfactory.
Preservative retention
Chemical analysis of the samples highlighted that the preservative retention levels were substantially below the minimum specifications required to effectively protect wood used in BS 8417 Use Class 4 ground contact applications where components are exposed to a severe biological hazard environment involving permanent ground contact and frequent wetting.
Conclusions
The gate posts failed prematurely due to insufficient preservative retention, despite satisfactory evidence of preservative penetration. This points to a treatment process control shortfall resulting in an insufficient preservative loading being achieved, rather than a failure of the preservative to perform as intended when correctly applied. This left the wood vulnerable to fungal decay at the groundline - the highest risk zone for deterioration in service - resulting in failure well before the anticipated service life.
Under normal circumstances, these findings would support a formal complaint or warranty claim against the supplier and/or the wood treater. In this case, a lack of documented evidence prevented the contractor from seeking redress.
Poor traceability prevented redress
An important lesson from this case was the importance of obtaining and retaining procurement and treatment records for the four gate posts.
The lack of evidence meant that the AFI contractor involved was unable to identify either the original supplier or wood treater who were culpable. As a result, no complaint, warranty claim, or supply-chain investigation could be pursued.
Consequently, the full cost of replacement and remedial works fell to the contractor - an outcome that most certainly could have been avoided through proper traceability and record keeping.
Exposing a cross-cut section of post to a copper reagent highlighted that preservative penetration of the vulnerable to decay pine sapwood was satisfactory...
Testing showed good preservative penetration but insufficient retention for UC4 performance, highlighting a treatment process control failure.
Key lessons learned
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Preservative loading/retention is critical. Adequate penetration alone does not ensure durability.
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Groundline exposure remains the highest risk area for wood decay and failure.
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Obtaining and keeping documented records is essential to evidence product traceability, quality assurance and protect against commercial damage.
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Multiple failures within the same installation are indicative of a systemic issue with the treatment process and not a failure of preservative chemistry.
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Procurement specifications in line with the British Standards and independent quality assurance schemes like the WPA Benchmark QAS play an important role in reducing the risk of premature failures.