The BSI committee responsible for wood preservation standard BS 8417 accepted the proposal relating to 30-year desired service life for sawn wood posts with exposed heartwood faces.
But no changes are being made at this time to strengthen the 15-year specification, with the WPA saying such a move would be "premature".
The WPA, preservative manufacturers and treater members had originally flagged up that longer service life could be compromised by exposed heartwood surfaces resistant to preservative penetration, such as Douglas fir. There has also been growing awareness about the variability of the natural durability of heartwood, especially in fast-grown plantation softwoods.
The new specification – to come into effect in 2014 – increases the preservative penetration class from NP5 to NP6 for wood with permeable sapwood (pine) for ground contact (Use Class 4). Wood with resistant sapwood has a new requirement for 6mm penetration of exposed heartwood.
So the specification for wood with permeable sapwood will be full sapwood and 6mm exposed heartwood penetration, with heartwood penetration visible in 75% of the cross-section of the heartwood analytical zone along any face in which heartwood is present,
Wood with resistant sapwood requires 12mm sapwood penetration and 6mm exposed heartwood penetration, with 75% visibility in heartwood penetration as above.
But the WPA has not proposed any strengthening of the 15-year desired service life specification for ground contact sawn timber.
"Although some failures of posts earlier than 15 years have been seen these have been attributed to inadequate preservative penetration and/or retention and in some cases incorrect installation practices," it said.
It also said 15-year material had smaller cross-sections compared with 30-year products, which limited heartwood exposure.
But the WPA plans to start a field study of posts sawn from home-grown spruce, pine and Douglas fir later this year which will inform future recommendations on ground contact timber treatment.
