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


  • How durable does the timber need to be for my project?
  • Is the timber species naturally durable?
  • If not, has it been pressure-treated to the right Use Class?
  • Has my supplier given me documentary evidence to prove it?
  • Have I treated any cut or notched timber?

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. European oak has been re-classified as Durability Class 2-4; if you are using it outdoors, make sure it meets Durability Class 2-3.
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.
Treated timber - fitness for purpose (Based on BS EN 335:1, which defines the treatment Use Class for different applications)
Use ClassService situationApplication examples
1Internal: permanently dryPitched roofs; floor boards; architrave; internal joinery; timbers in upper
floors not built into solid external walls.
2Internal: occasional wettingTiling battens; frame timbers in timber frame houses; timber in pitched roofs with high condensation risk; timbers in flat roofs; ground floor joists; sole plates (above dpc); timber joists in upper floors built into external walls.
3c (coated)External: out of ground, frequent wettingExternal joinery; roof soffits, fascias and bargeboards; cladding, valley gutter timbers; external load bearing timbers.
3u (uncoated)External: out of ground, frequent wettingFence rails and boards; gates; agricultural timbers not in soil contact;
landscaping and decking timbers not in contact with the ground.
4External: in permanent ground or water contactFence and deck post; gravel boards; agricultural timbers in soil; earthretaining walls; poles; sleepers, playground equipment; lock gates; jetties and boardwalk support.
5External: in permanent ground or water contactMarine piling, piers and jetties, dock gates, sea defences, ships hulls.

What’s the right treatment Use Class for the job?

1Roof timbers (dry)1
2Roof timbers (risk of wetting)2
3Tiling battens2
4Barge boards, fascias, soffits3c
5Timber frame components (except sole plates)2
6Frame sheathing (plywoods)2
7External cladding3
8Battens for external cladding2
9First floor joists1
10Ground floor joists2
11External joinery3c
12External doors3c
13Sole plates above dpc2
14Sole plates below dpc4
15Decking (out of ground contact)3
16Decking (in ground contact)4
17Fence posts4
18Fence panels3
19Garden products3-4
20Garden products (in water contact)4
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 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.

Treatment processes and preservative chemicals

The chemicals used in wood preservatives comply with current EU regulations. They contain specifically targeted biocides designed to present a minimum hazard to the wider environment. There are two main types of pre-treatment process, 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, it provides a 15 to 60-year service life. The preservative is forced deep into the cellular structure of the timber, typically giving 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 to deliver a 30 to 60-year service life. Treatment provides an effective ‘envelope’ protection around the timber and leaves the colour virtually unchanged. A colour indicator, as well as waterrepellency, can be added to the treatment if required.

Further information and advice

Other relevant Timber Trade Topics:
  • Cladding
  • Decking
  • In the Garden
Find out more information at including a free download of the WPA manual
Industrial Wood Protection

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.