UC4 treated timber refers to wood that has been treated to a standard that makes it suitable to be in permanent contact with the ground outside, for example timber posts or decking joists.
To achieve this level of durability the timber goes through a complex treatment process. It is first kiln dried, reducing the moisture content of the wood to a maximum of 28%, allowing the subsequent application of preservative treatment to penetrate deeper into the timber. Kiln drying is a long and controlled process, taking up to a week, to ensure that the wood does not split or warp during drying.
The dried timber is then pressure treated, which is the method used to force the preservative deep into the wood. To achieve this the timber is loaded into an air-tight vessel and a vacuum is applied. The vessel is then flooded with preservative at pressure while still under vacuum, forcing the treatment into the timber. Excess preservative is then pumped out, the vacuum is released, and the returning air pulls the preservative even deeper inside the timber.
The treated wood is then allowed to air dry for 48 hours, resulting in a highly durable and water resistant timber which should give at least 15 years service for ground contact applications.