What is Use Class 4 treated timber?
Use Class 4 is the preservative treatment level required for timber in ground contact or permanently wet conditions, which means fence posts, decking joists and gravel boards. It is specified under BS 8417. Anything going into the ground should be Use Class 4 and pressure treated, not dip treated, and every cut you make on site breaks the treatment and needs resealing.
The use classes describe where a piece of timber will live. Use Class 4 covers timber in contact with the ground or fresh water, frequently wet or providing exterior structural support, and that is exactly the life of a fence post. Timber processors incise posts before treatment so the preservative penetrates rather than sitting on the surface, and the treatment specification is governed by BS 8417, which matches the process to the species, the component and the intended service life.
Pressure treatment and dip treatment are not the same thing and the difference is not cosmetic. Pressure treatment forces preservative into the timber under vacuum and pressure; dipping wets the outside. Manufacturers who dip say so in their own guarantee terms: dip-treated products must not be installed in direct ground contact, must sit above a pressure-treated gravel board, and need retreating with a preservative every year. A dip-treated panel is a fine fence and a terrible post.
The detail almost everyone gets wrong is cutting. Treatment forms an envelope, and the moment you saw a post to length or drill it, you have opened raw timber at the most vulnerable point, usually the end going into the ground. Manufacturers exclude cut, drilled or notched in-ground timber from their guarantees for exactly this reason. Every cut we make on site gets end-grain preservative brushed on before it goes anywhere near a hole.
What to ask for on a quote: Use Class 4 pressure-treated timber for posts, gravel boards and anything in ground contact, and confirmation that cut ends are treated on site. It is a short sentence that separates a fence built to last from one assembled to look finished on the day.
More in materials and lifespan, or browse all 36 questions on the FAQ index.
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