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Wingham Timber, Kent
We use a different specific treatment cycle for each Use Class and each different species of timber we pressure treat to meet the requirements of BS8417.
For User Class 1 where the timber is permanently dry, you are only treating for insect attack and therefore it is more of a coating to stop the insects burrowing into the wood. For User Class 2 and above the amount of penetration required increases in-depth as the risk of moisture increases to ground contact materials where permanent wetting can be an issue.
Different species accept different amounts of treatment. Some species (eg whitewood spruce) do not allow treatment to go deep into the wood even under high pressure. With the aid of incising these timbers can meet the requirements of User Class 3 and User Class 4.
In these species the depth of treatment (to meet UC4 requirement is only 6mm) may not penetrate all the sapwood (the sweet area of the tree vulnerable to attack when oxygen and moisture is present).
Splits in the timber, nails, fixings may well go in deeper into the sapwood than the treatment and breakdown may follow. Radiata pine treated on a specific cycle gives FULL SAPWOOD PENETRATION (NP6).
Certain species (redwood pines) need to be kiln-dried to below 28% moisture content so that the timber accepts a better penetration of the sapwood. We import a Kiln Dried European Radiata Pine, a species that accepts very high levels of treatment when treated on the correct cycle.
We pressure treat on a specific cycle at a higher chemical concentration to reach User Class 4 with a 30-year desired service life for full sapwood penetration. Link to WPA Use class 4.
Unfortunately, due to lack of knowledge, a high percentage of posts, sleepers, path edgings, gravel boards, etc sold have not been treated for ground contact. This year the Wood Protection Association has teamed up with other organizations to promote the correct treatment for the end purpose but there are still large quantities of these items being sold for the wrong end purpose. As we import the Kiln Dried Radiata Pine, treat it on-site and sell it to both the supplier and the end-user we may still be cheaper for the correct product rather than other companies selling the wrong product.
We often get told that posts do not last as long as they used to. In 2004 arsenic compounds were no longer allowed to be used for most end uses due to their leaching.
Since then, the treatments are not as effective on all softwoods without help. Since 2004 a high percentage of posts, sleepers, etc sold have not been suitable for ground contact use.
Certain species need to be incised to reach a standard UC4 level. Kiln-dried Radiata Pine has a deeper penetration without incisions.
We treat our Radiata Pine on specific treatment cycles and our penetration is NP6.
This means there is 100% sapwood penetration. The sapwood is the main area where rot and insect attacks take place.
We pressure treat on-site with Celcure AC500 @ 3% solution strength on a specific UC4 Radiata Pine cycle for Use Class 4 application and the desired service life of 30 years.