| 55
ratios and treatability characteristics can result in differing levels of achievable service life.
“Take plantation-grown pine and spruce,” he said. “Although having the same natural durability classification, pine sapwood accepts preservative penetration more readily than spruce, which is important when buyers of fence posts are looking for long-term performance.”
PRESERVATIVE DEVELOPMENT Wood preservatives contain active ingredients known as biocides that are strictly regulated. Manufacturers point out that developing an effective preservative is, by necessity, a long, drawn-out process. “A multitude of factors need to be addressed,” said Mr Ryan who explained that product development begins in a laboratory, working with ingredients identified as suitable, testing their effectiveness against the biological organisms they will face in service and ensuring their formulation stability. Large scale efficacy tests follow with field trials and checks that everything works on an industrial scale. If successful, the product is commercialised. This involves gaining registrations and approvals in the different countries where it will be sold, which necessitates a raft of supporting data. “The financial and time investment in ensuring a preservative works is huge,” said Mr Ryan.
TREATMENT PROCESS AND QUALITY VERIFICATION
“Demand for independent quality verification of treated wood in the UK is growing” said Mr Ryan who explained that the move is driven by Timber Development UK (TDUK), the trade body whose members account for over 80% of all timber sold in the UK. Treaters seeking verification of their treatment capabilities can subscribe to the WPA Benchmark Quality Assurance Scheme. Certification is awarded to treaters who demonstrate their quality systems and processes are reliable and robust. “WPA Benchmark compliance involves auditing every aspect of the treatment process,” said Mr Ryan, adding that the audit covers an assessment of the incoming wood quality, moisture content, preservative management, treatment cycles and measurement of results. Sample analysis of treated products is required to demonstrate compliance with the preservative penetration and retention requirements of the scheme.
CORRECT TREATMENT SPECIFICATION Whilst one piece of timber may look very much like any other, the level of preservative protection could be very different, making it inappropriate for some end uses. By law, any product offered for sale must be fit for its intended use. Despite this obligation, the
WPA says treated wood is often seen for sale without any reference to its end use suitability. Recent supply chain research by the WPA confirmed that the term ‘green treated’ is still common in sales materials. “More worryingly, that same WPA research reveals the sale of Use Class 3 fence posts alongside Use Class 4 posts,” said Mr Ryan. Use Class 3 treatment applies to timber used externally above ground. WPA is emphatic that this is a big mistake and incorrectly treated fence posts are destined to under perform and negatively impact the reputation of all treated timber.
British standards for the impregnation of wood (including the application of the Use Class system) are set out in BS8417 (Preservation of wood – Code of practice) and in the WPA Code of Practice – Industrial Wood Preservation. These have been available for decades, yet the WPA is finding vague and incorrect specifications for treated wood still evident in the supply chain.
“This only serves to encourage bad practice and is often perpetuated in public tenders for fencing materials by organisations who should know better,” said Mr Ryan.
INSTALLATION GUIDANCE
Good installation practice helps significantly in ensuring the long-term performance of a correctly treated fence post. Practices such as cross-cutting and notching can damage the treatment envelope, exposing untreated parts of a post and compromising service life – end grain wood preservatives should be used. Best practice guidance on installing round and square sawn preservative treated fence posts is available in WPA Guidance Note TW13 which is free to download at
www.thewpa.org.uk
BUILDING CONFIDENCE Working with TDUK and other timber trade bodies, the WPA has intensified efforts to raise awareness about the correct specification of treated wood, particularly for Use Class 4 ground contact applications. A recent WPA survey shows that the
‘Make sure it’s 4’ message is getting through to buyers – unprompted awareness about treatment use classes is up significantly said the organisation, which has a mission to build confidence in treated wood performance. Despite this the Association of Fencing Contractors (AFI) continues to express concern about treated wood performance. “The WPA has agreed to support AFI contractors by investigating posts that have not met service life expectations,” said Mr Ryan. “The aim is to identify the root cause of under performance to help all contractors buy timber fence posts they can trust.” Building confidence in treated wood is vital to growing demand for added value treated wood products say WPA and TDUK. To achieve this, both organisations are promoting the following guidance:
• If you’re buying treated timber, look for third-party accreditation, such as WPA
Benchmark and always ask for documented proof of treatment.
• If you sell treated timber, ensure it is fit for purpose by sourcing from a WPA
Benchmark approved treater.
• If you treat timber, show you’re a trusted, quality assured supplier by attaining
Benchmark Approved Treater status.
PROOF OF PERFORMANCE The WPA has tangible evidence that treated wood does work in ground contact. In 2015 it commissioned BRE to establish a 15-year field trial. BRE installed 1,360 fence posts of pine, spruce, larch and Douglas fir and 300 (EN252) test stakes across two field sites with differing soil types, one in Watford and one in Scotland. BRE’s seven-year assessment of both sites reported little sign of deterioration of all treated posts, with incised spruce posts showing no signs of deterioration. In comparison, BRE highlighted that untreated wood references used to indicate the severity of the exposure risk at each site were failing. “This field trial is a case of a trade body practicing what it preaches,” said Mr Ryan, adding that the WPA is confident that the results will clearly underpin its belief that treated wood, when correctly specified, treated and installed will provide a long, trouble-free performance. ■
Above: Neil Ryan provided technical input at the workshop
*Neil Ryan is a wood preservative research scientist, business manager and consultant in the timber treatment industry. He is on the board of the WPA and is chair of the Associations Technical & Regulatory Affairs Committee.
www.ttjonline.com | May/June 2023 | TTJ
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85
