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De-mystifying the timber Use Class system

The timber Use Class system, as defined in BS EN 335-1 is used across the timber industry to determine how your timber should be treated so that it stands up to the job in question. Specifying the correct Use Class number is an important step and your service provider should be able to give you full guidance prior to treatment.

What will it be used for?

Quite simply, the eventual classification will be dependent upon how and where you intend to use the timber. The various classes are based on the potential threat level to the timber of decay or insect attack from its eventual use. So, if you are going to use a piece of timber indoors, in a dry environment, the risk of decay or attack is far lower than if you used it outside and perhaps in the ground. The higher the Use Class number, the higher the likely risk and the greater the treatment therefore required.

timber use class system

As easy as 1 to 5

The easiest way to illustrate this is with a simple list of examples allocated to each Use Class number:

Use Class 1: This is suitable for dry, internal timbers such as upper floor joists and truss rafters. The main risk is from insect damage and timber treatment will manage this risk effectively for the service life of the timber.

Use Class 2: Still only suitable for internal timbers, this level would indicate that the timber might be at a slight risk of wetting. For example tile battens and CLS framing (external walls).

Use Class 3a: This is the first level for timber to be used outdoors. It will be coated however and above ground – so window frames would be the best example in this instance.

Use Class 3b: Uncoated, external timbers that remain above the ground would be treated to this level. Consider fence rails as the perfect example.

Use Class 4: If direct soil or water contact is intended, the timber must be treated to Use Class 4. This would include fence posts and certain parts of a decking structure that will be sunk into the ground.

Use Class 5: The highest level possible, this is required for marine use timbers such as marine pilings, where ground and water contact is continuous.

Learn more about our timber treatment services

 

Looking at the timber 

timber use class system

Here’s a very useful image that demonstrates what happens to your timber when it is pressure treated. The light area in the centre is the heartwood of the tree – naturally durable, it does not take up any treatment. The sapwood can be seen as the outer, dark area. This is the new growth and not naturally durable. This is the part that will rot over time if left untreated. The amount of chemical retained in the wood is what determines the Use Class level achieved during treatment.

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