UK grown Sitka spruce, larch and Douglas fir are the focus of a Building Research Establishment (BRE) research project that explores the incising process as a means to increasing the end use applications of these timbers. Funded by the Forestry Commission and Scottish Enterprise, the project looks at improving the penetration of preservative treatments in the timbers by incising in order to increase durability.
The three species are, in many ways, excellent construction timbers that are relatively cheap, readily available, have adequate strength characteristics, are easy to work, have a relatively low permeability to water and are sustainably grown. However, they are also susceptible to fungal and insect attack.
Impregnating Sitka spruce with wood preservatives often achieves relatively poor and uneven penetration and therefore offers only modestly enhanced protection. While preservative treated spruce is used extensively in the UK, applications are limited up to Hazard Class 3 (EN335-1). This includes applications such as highway fence rails, but does not include fence posts or other in-ground applications (Hazard Class 4) which currently provide a sizeable and lucrative domestic market.
North American practice
Incising timber by creating small slits or pin holes in the surface of the timber to improve uptake and penetration of preservative has long been recognised as an accepted method for improving the durability of timber. In the US and Canada it is a routine technique for improving preservative penetration and retention, but is used to a much lesser extent in Europe.
Incising is not currently practised in the UK, largely because the benefits have not been shown to justify the initial outlay on equipment. And criticism levelled at incising is based on the reduction of bending properties; research has shown that the modulus of rupture was reduced by 16% and the modulus of elasticity was reduced by 13% due to incising. However, these initial strength losses are not significant for many end uses and the increase in preservative penetration provides a significant extension of service life that far outweighs the initial loss of strength. In the US and Canada commodities are engineered and specified to accommodate the slight strength loss incurred by incising.
BRE data from an ongoing field trial (established in 1972 and reported in BRE Information Paper IP14/01) shows that in-ground performance of incised and treated spruce posts far exceeded traditional expectations – after 30 years of the trial no incised posts have failed. The incising prior to preservative treatment has doubled the service life of the treated posts.
Hazard Class 4
Based on this information, the Forestry Commission and Scottish Enterprise asked BRE to explore how incising techniques can be exploited to take treated Sitka spruce into Hazard Class 4 applications such as highway fence posts, landscaping timber and decking. In addition, incising might provide extended service lives for Douglas fir and larch in these conditions (both timbers are used as highway fence posts) as well as more consistently treated material that is easier to assess for quality control purposes.
BRE has recently completed trials that involved incising the surface of 1,000 battens of these three timber species prior to preservative treatment in order to:
Two different incising patterns were explored, the original BRE pattern used for the 1972 field trials and a new pattern based on latest practice. These were evaluated alongside unincised sawn timber. Interim results have concluded that:
Positive response
These findings have been presented to the woodworking industry, preservative manufacturers and timber retailers for their comment on the consistency of the preservative treatment and appearance of the incised timber. Many preservative manufacturers were impressed by the results, as were retailers who are keen to stock UK-grown timbers.
Many sawmillers could see the market opportunities that incising offers, particularly with respect to providing a clear demonstration that the penetration requirements of BS 8417 (2003) have been met. Others were concerned about the added cost of the process, customer perceptions on the visual effects of incising and whether the timber could really compete against cheap import timbers for the same applications. BRE estimates that after initial set-up costs the incising process adds £5/m3 to costs in additional electricity and wood preservative solution.
The customer perceptions of the incised product are being investigated further as are the opportunities for marketing based on a “long service life – quality product” platform to stimulate competition with imports.
Further trials
In the next phase of the project, incised posts will be installed in ground contact in the BRE test field alongside the original national field trials. Data from this field trial will be essential to confirm the performance predicted from laboratory data and to evaluate the opportunities for tailoring preservative concentrations to deliver more precise service lives. A demonstration fencing system made of UK-grown Sitka spruce will be produced in collaboration with a sawmill and timber treater. Examples will be installed at various sites around the country with regular assessments of performance.
A BRE best practice guidance document for incising will be published towards the end of 2004. This publication will provide a platform for increasing the end use applications of UK home grown Sitka spruce, Douglas fir and larch.
