Which treatment use class do I need for my timber?

This is the single most useful thing to know when buying timber for the garden, and it is almost never volunteered. A UC3 rail and a UC4 post look identical, cost differently and have completely different lives in the ground. The classes come from a European standard and every reputable treater marks its stock with the class it has treated to.

Step by step

1. Work out the exposure honestly.Not what you hope, what it will actually get. Is the timber inside and dry, inside but liable to get wet, outside but above ground and able to drain, or in contact with soil, gravel, standing water or a permanently damp surface?
2. Match it to a class.UC1 — internal, dry, no wetting risk: internal studwork and joinery. UC2 — internal with an occasional wetting risk: roof timbers, timber in a damp space. UC3 — external, above ground: fence rails, cladding, decking boards, gates, garden furniture. UC4 — in ground or fresh water: posts, sleepers, retaining boards, anything buried or permanently damp.
3. Buy UC4 for anything that touches the ground.This is the decision that matters most. A UC3 post in the ground is the classic failure: it snaps at the soil line after a few years while the panels either side are fine. Ask for ground-contact treated stock specifically.
4. Ask the merchant which class the stock is treated to.It is a fair and normal question and staff will usually know or can check the pack label. If nobody can tell you, assume it is the lowest class in stock and buy elsewhere for anything going in the ground.
5. Treat every cut end regardless of class.The class describes the factory treatment of the piece as supplied. Cutting it locally reduces it to untreated timber at that point. Brush end-grain preservative into every cut, notch and hole.
6. Do not rely on class to solve a bad detail.Timber that is permanently wet because water is trapped against it will eventually fail whatever it is treated to. Keeping water off and letting the piece dry does more than a class step ever will.

Tips

• Decking boards are normally UC3, and decking joists in contact with the ground or a damp base should be UC4. Mixing them up is why the frame under a deck fails before the boards do.
• Some suppliers use their own colour codes for classes — brown for one, green for another. That is a supplier convention, not a standard, so read the label rather than the colour.
• Treatment guarantees usually name a class and a period. If a supplier offers a long guarantee, ask which class it applies to, because it will not apply to a UC3 piece used in the ground.

Common mistakes

• Buying "treated" without asking which class — Most merchant garden stock is UC3, which is right for rails and panels and wrong for posts. The word "treated" on a label covers both.
• Using treated fence timber structurally without checking the strength grade — Treatment and strength grading are separate specifications. A UC4 sleeper has been treated for ground contact and may have no strength class at all, which matters if it is holding something up.

Questions people ask