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What is pressure treated wood?

Pressure treatment is a term that is used to describe the process of forcing chemical preservatives into timber under vacuum pressure, in order to protect from deterioration such as insects, fungal decay and rot.

Learn more about WJ Group’s timber treatment services in Hull and Rochester

To create pressure treated wood, the timber is placed into a large metal tank. The correct chemical formulation and water is then pumped into the tank so that the timber is totally submerged. Pressure is applied by way of a vacuum and upon its release, the liquid is pumped out of the tank and the timber is removed.

pressure treated woodThe image demonstrates how the vacuum pressure works. The central part is the heartwood and it does not take up any of the preservative – it is naturally durable. The outer area is the sapwood and this is the part that will rot if left untreated. This wood takes up the preservative chemicals and the amount of chemical retained determined the Use Class classification achieved.

Watch our explanatory video on timber treatment

The freshly pressure treated wood now needs to dry out prior to any further use, so that it can reach the optimum moisture content.


What treatment does my timber need?

This depends entirely on how you are going to use the pressure treated wood after treatment.

Use our nifty tool to determine what treatment you require

At WJ Group, we pressure treat wood to Use Classes, depending on the end usage. This is an industry recognised system and we can offer warranties on some of our treatment services.

Here’s how it works:

Use Class 1: suitable for internal timbers that will remain dry. For example upper floor joists and truss rafters.

Use Class 2: suitable for internal timbers at risk of some wetting. This might be tile battens or CLS framing (external walls).

Use Class 3a: suitable for outdoor timbers that are coated and to be used above ground. For example window frames.

Use Class 3b: suitable for outdoor timbers that are uncoated and to be used above ground – such as fence rails.

Use Class 4: suitable for soil or fresh water contact timbers. This might include items such as fence posts that will partly go into the ground.

Use Class 5: suitable for marine use timbers such as marine pilings.


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