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Spruce (Kiln Dried) incised and treated posts inspected at the Birnie Wood site in year 7, showed no signs of deterioration. 

WPA Field Trials Year 7 Summary

Treated wood fence post field trial extended

10 April 2024

Back in 2015, the Wood Protection Association (WPA) commissioned an independent 10-year field trial of sawn, pressure treated British softwood fence posts. The results so far have been very positive. The WPA now intends to extend the trial until 2030 to gain further long-term insight into the performance of treated wood in ground contact. 


The initial purpose of the field trial was to test the performance of correctly specified and treated, Use Class 4 ground contact fencing timbers – using different species and preservative treatment formulations. The results would inform a number of objectives:


  • Gather data on the efficiency of preservative penetration and retention 

  • Update generic timber treatment specification guidance in BS8417 and the WPA Code of Practice: Industrial Wood Preservation

  • Support the WPA Benchmark quality approval scheme for treated wood

  • Provide robust mean and median life data for each treatment/post combination

  • Build confidence in UK sourced and treated wood used in ground contact applications


The inspections in years 5 and 7 were very encouraging. The fact that no significant or only minor signs of deterioration were observed in certain treated Sitka spruce (Kiln Dried), larch and Douglas fir posts, provided confidence that the other preservative treated material (pine) was performing even better.


As the field trial approaches year 10, the board of the WPA feel it vital to continue the project for a number of reasons – not least of which is to provide the timber industry with credible, real time evidence that copper-based preservatives are effective when correctly specified and applied for ground contact use. It is the support of sponsors that is enabling the WPA to maintain the study for 15 years. 


There will be a full inspection at both field trial sites (Garston in England and Birnie Wood in Scotland) in year 10 (2025). The results will be published, as we have in years 5 and 7, with sponsors and WPA members being first to be informed.Maintaining inspection activity at each year from then until 2030 is important as we expect most differentiation between species, treatment and incising to materialise in the next five years. WPA confirms that it has commissioned BRE to continue monitoring and reporting on the field trial until its conclusion in 2030.


There are now additional significant objectives we would like to achieve from the findings of the study, these are to:

  • Demonstrate that treated incised spruce is capable of meeting a 15-year service life

  • Demonstrate that treated pine, Douglas fir and larch are capable of meeting a 15-year service life

  • Understand the impact of incising on the performance of treated heartwood


The intention is also to add a number of ‘off-the-shelf’, mystery shopper bought posts – as a direct comparison to the original purposely produced samples.  It will be interesting to note the performance of what is actually being sold and how it is marketed and labelled – especially in light of some merchants still offering Use Class 3 treated posts. It may focus attention for buyers on the need for correct specification as well as the value of independently accredited treated timber.

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