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When to use high pressure timber treatment

Knowing what specification of timber treatment you need can be complex and determined by a number of factors ranging from timber species to end usage and environmental factors. Furthermore, there are a number of terms banded around that can make the situation seem rather unclear: do you need high pressure timber treatment or low pressure? Is that the same as tanalised timber? And what’s Wolman timber treatment?

At WJ Group, we live and breathe timber treatment and we want to ensure that you select the correct specification for your own particular project or usage requirement. Therefore, we’ll always offer as much advice as necessary to our customers, who might possess varying levels of knowledge about timber treatment.

high pressure timber treatmentWhat is high pressure timber treatment?

The pressure treatment of timber is undertaken in large cylindrical tanks. The packs of timber are first placed into the tanks which are then sealed so that vacuum pumps can remove all the air. The tank is then flooded with the specified water based preservative chemical. A pressure of 12 bar is applied, forcing the preservative chemicals into the timber. The further into the timber the chemicals penetrate, the higher the Use Class. High pressure timber treatment generally describes timber that is treated to Use Class 3 and above.

Learn more about Wolman timber treatment

How does the Use Class system work?

An industry recognised standard, the Use Class system applies a scale to the level of pressure treatment, ranging from low to high (1 to 5). How and where you wish to use the timber will determine what Use Class level it should be treated to:

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.

pressure treated timberHigh pressure timber treatment from 3 and above

As soon as the timber is to be used outside, it needs to be high pressure treated. If it’s going to come in contact with the ground, opt for Use Class 4 to maximise the life of the timber. Treating to the optimum level will ensure that the timber is fit for purpose and ready to give you a suitable service life to match your expectations. It also ensures that you don’t over specify treatment and spend more than you need to. Use Class 4 is a significant benchmark and one to take careful note of.

All those terms

Finally, here are all those various terms explained quickly. There’s no need to be confused when considering high pressure timber treatment or any other form of timber treatment. We’re a specialist service treater and happy to help you determine the right way forward for your business.

Pressure treated timber: timber that has been treated in a sealed tank with a water and chemical mix, and a vacuum pressure forcing the chemical into the timber.

Tanalised timber: often mistaken for a generic term for pressure treated timber, but actually a trademark of Arch Timber Treatment, relating to their brand of pressure treated timber.

Vacuum treated timber: you might guess that this is the same as pressure treated timber.

Preservative treated timber: again, somewhat similar to pressure treated timber, which serves to protect the timber from insects, wood destroying fungi and other biological decay hazards.

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