How to get the best results from preservative-treated timber

How to get the best results from preservative-treated timber

Not all treated timber is the same. David Hopkins, CEO of Timber Development UK, offers some advice on how to make sure you get the best results from preservative-treated timber.

As a builder, you know what makes a good piece of timber. When you’re in a merchant’s branch picking out the materials for your next job you know the look you want, the size you need, and you know the differences between the types of timber available.

But do you know the difference between a Use Class 2 treated timber, and one that’s been treated to Use Class 4? And do you know why it matters?

The industry often uses the word timber to refer to any piece of wood that has been prepared to use in a building or on a construction site, but not all timber is the same. Some species are more durable, some have a longer working lifespan, and others are more stable and less prone to shrinking or warping over time.

Some timber species are more readily available than others, and timber suppliers often extend the useful service life of these species through the use of preservative treatments. Many softwood components used in modern construction, for example, are treated with preservatives during the manufacturing process.

Timber for internal or external use

Some of these treatments make the timber suitable for external use, while other treatments are designed for internal use only, and it’s important that you understand the differences and buy the right treated timber for your project.

Using the wrong treated timber can have very serious consequences in terms of safety and durability, especially when you use the wrong timber outside and in structural applications, such as fence posts, deck posts, joists and beams.

Wood Protection Association Chief Executive Gordon Ewbank explains: “Preservative treatment provides wood with added durability. While one piece of treated wood may look very much like any other, the level of preservative protection could be very different. That’s because the British Standard for wood preservation – BS 8417, requires that the loading and penetration of preservative, impregnated into the wood, is tailored to the desired end use.”

Use Classes

The British Standard for wood preservation, BS 8417, groups the uses of timber into various ‘Use Classes’. Three of the most common Use Classes found in construction products are Use Classes 2-4, which are described in BS EN 335. These are:

  • Use Class 2 – (Above the ground or DPC, covered): covers timber for interior use in a dry environment, such as battens, framing, joists, roofs
  • Use Class 3 – (Above the ground): covers timber for exterior applications above ground, such as deck boards, cladding, and fence rails. Use Class 3(u) indicates uncoated wood, while Use Class 3(c) indicates coated wood, such as on a painted timber window.
  • Use Class 4 – (In-ground contact): covers wood used on, or in, the ground and for external structural support, such as fence posts, deck posts, joists and beams, whether or not it sits on other materials, like an under-deck weed-suppressing membrane.

To make it easier for you to identify which is the right timber for your project, Timber Development UK members who supply preservative-treated softwoods have committed to clearly marking the relevant Use Class application on both their sales and delivery notes, and their invoices – and we’re encouraging builders’ merchants to clearly mark the relevant Use Class in their branches as well.

Some producers may offer a warranty on their timber if it is installed under certain conditions and for a specified use, but not all treated timber comes with a manufacturer’s warranty, so it is always best to check – and if you do need a warranty, always get confirmation in writing of the likely performance of the timber when used for your intended application.

No more green treated timber

Previously, you might have seen timber being described as ‘green treated’, but this phrase does not give you enough information to know whether it is treated for use under Use Class 2, 3 or 4. So if you’re in a builders’ merchant and the timber in the yard doesn’t have its Use Class clearly displayed, speak to the merchant staff and check exactly what treatment has been applied before you make a purchase, otherwise you risk using the wrong timber.

Flame-retardant treatments

Flame-retardant timber treatments must always be applied in a factory-based, quality-controlled environment that adheres to strict standards – never on site. They can either be applied to products during the manufacturing process or incorporated into the materials used to make wood-based products, such as MDF and OSB.

Timber Development UK and the Wood Protection Association have published a range of guides and leaflets to help builders understand the different levels of preservative protection and where each should be used. These resources are free to download from

There is also a free online training presentation that runs through the basic facts at

Related posts