Treated Timber

Wood Campus Timber Trade Topics are produced in collaboration with Timber Development UK and the industry technical expert on the subject, in this case, the Wood Protection Association. For further information, visit


  • What Use Class timber is needed for the project?
  • Do I have evidence to prove the material is treated to the right level?
  • Do I have guidance available on the safe and effective use of treated wood?
  • Is any cut or notched timber sure to be re-treated?

Preservative treatment provides wood with added durability. However, it’s a mistake to assume that all pressure treated wood is the same. Whilst 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 Class 4
Ask your supplier whether the timber has been treated appropriately for its end use. Timbers destined for Use Class 4 situations will be structural or permanently exposed to wetting in either ground or fresh water contact. For optimum durability it is important to ensure the correct specification has been used. Make sure you ask for timber treated to Use Class 4.

How naturally durable are timber species?
Many hardwood species are naturally durable and can be used outdoors untreated, but they are expensive and supplies of certified timber are limited. Some softwoods are relatively durable, but most will need preservative pressure treatment if used outdoors or in humid conditions. They are inexpensive, and certified supplies are plentiful. Modified timbers, such as Accoya™, provide durability with sustainability, but are expensive and may need specialist stainless steel fixings.

Cutting or notching will expose untreated timber, which should be treated with a generous coat or two of brush-applied end-grain sealer or preservative.

What’s the right treatment Use Class for the job?
Ask your supplier for evidence that the timber has been treated appropriately for its end use.

Treatment processes and preservative chemicals
The chemicals used in wood preservatives comply with current EU regulations. They contain specifically targeted biocides that are designed to present a minimum hazard to the wider environment. There are two main types of pre-treatment processes, both carried out by timber suppliers, merchants or joinery companies, in enclosed and strictly controlled industrial vessels.

Vacuum, high-pressure treatment
Suitable for the full range of end uses, but particularly for external applications, both in and out of ground contact. The preservative is forced deep into the cellular structure of the timber, which generally has a green tint. Additives can give either a rich brown colour, usually for fencing and landscaping timbers, or extra water repellency for decorative external timbers, such as decking and cladding timbers.

Double vacuum, low-pressure treatment
Used for building and joinery timbers in Use Classes 1, 2 and 3c. Treatment provides an effective ‘envelope’ protection around the timber and leaves the colour virtually unchanged. A colour indicator, as well as water-repellency, can be added to the treatment if required.

Sustainable timber
Timber is the most sustainable mainstream building product. It is naturally renewable. Over 90% of timber used in UK construction comes from Europe, where more trees are grown than harvested (source: TTF Statistical Review 2016).
Softwood and temperate hardwood forests in Scandinavia, Europe, Canada and North America are stable or growing. Growing forests act as carbon sinks; wood products act as carbon stores.
Ask for PEFC or FSC Chain of Custody certification.
See Wood Campus RIBA CPD module Procuring Sustainable Timber for more on timber certification and sustainability and government requirements.