The timber Use Class system
When choosing timber products, especially for outdoor use, it is essential to select the right grade of timber for the right job. The Use Class system is a British Standard classification of wood treatments that has specific treatment criteria that timber must meet for various purposes.
- UC1 - wood that is used indoors with no chance of water contact
- UC2 - wood that is used indoors with chance of occasional water contact e.g. bathrooms
- UC3 - wood that is used outdoors, above ground and exposed to the weather e.g. fencing panels
- UC4 - wood that is used outdoors, in permanent contact with the ground e.g. fence posts, decking joists
- UC5 - wood that is regularly or permanently submerged in seawater
What is UC4 timber?
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.
What is UC3 timber?
UC3 timber is suitable for outdoor use when not in constant contact with the ground, for example fencing panels or decking boards. It may have been pressure treated as above, but the penetration of the preservative is not as deep, meaning that the timber will require additional applications of treatment during its lifespan.
However UC3 timber should still have a service life of at least 15 years in situ.
Treating cut surfaces on UC3 and UC4 timber
The pressure treatment used to produce UC3 and UC4 timber leaves a continuous layer of preservative around the outside of the wood. However, should you need to cut the timber then the cut surface will break this layer and expose the untreated wood inside. In this case it is essential that any cut surfaces are treated with an end grain treatment to preserve the integrity of the treated layer and to prevent any water damage.
If you need to cut a UC4 fencing post, always make sure that the cut (and re-treated) end is above ground. In addition if cementing the fencing post into the ground, create a small area of raised concrete around the base of the post, rather than a dip in which water could collect.
Never rip saw treated wood along its length - a cut surface of this size will never be adequately sealed by an end grain treatment.
What is tanalised timber?
You may see timber referred to as tanalised or tanalized - this is exactly the same as pressure treated wood, but the brand of preservative used is Tanalith™. Always check whether your tanalised timber is UC3 or UC4 depending on its use.
What is C24 timber?
Kiln dried premium C24 timber is carcassing timber that has been graded to a higher level than the more common C16 timber. C24 wood is stronger and has smaller and fewer knots, sourced from slow growing trees with a tight ring structure, giving a uniform and durable structure to the wood. All of our decking joists are produced from C24 timber.
Ask the experts
For further advice about your project, or on which timber to choose, don’t hesitate to speak to our experts at your local AVS Fencing & Landscaping Supplies branch. Our ranges of pressure treated timber are all available from your local AVS Branch or for home delivery within our AVS Branch delivery area.