Guide
Treated timber use classes explained
Use classes under BS 8417 describe where treated timber can be used: use class 1 and 2 are internal, use class 3 is outdoors above ground, use class 4 is in contact with the ground or fresh water, such as fence posts, and use class 5 is in contact with sea water.
Written by the SK Trade team. Last reviewed 1 November 2026.
The classes
| Use class | Situation | Examples |
|---|---|---|
| 1 | Internal, dry | Internal joinery |
| 2 | Internal, risk of occasional wetting | Roof timbers, floor joists |
| 3 | External, above ground | Cladding, fence rails, decking boards |
| 4 | In contact with ground or fresh water | Fence posts, decking joists near the ground, sleepers |
| 5 | In contact with sea water | Marine and coastal structures (specialist treatment) |
Colour is not a guide
Treated timber can be green, brown or clear depending on the preservative. The colour fades and says nothing about the level of protection; the use class does.
Cut ends
Cutting exposes untreated wood. Brush a suitable end-grain preservative onto cut ends, especially for external timber.
Fixings
Use galvanised, coated or stainless fixings with treated timber outdoors so they do not corrode.
Related products
Related questions
Is your carcassing timber treated?
We supply treated carcassing as standard for structural work. Tell us if the timber is for external or ground-contact use so we supply the right treatment level.
How many roof tiles do I need?
It depends on the tile and its gauge (the spacing of the battens). Manufacturers quote tiles per square metre for each product; send us the roof area and the tile and we will work it out with ridge, hip and verge pieces.
Sources
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