What is a use class for timber?
A use class states how severely a component — usually timber — is exposed to moisture, and therefore to attack by fungi and insects. The higher the number, the harsher the conditions. The classes used to be called hazard classes, and in Dutch the term risicoklasse is still widespread. Crucially, the class describes the situation the timber sits in, not the quality of the timber itself.
The classes
| Class | Situation | Example |
|---|---|---|
| 1 | Interior, dry, permanently covered | Floor joists, internal doors, stairs |
| 2 | Interior or covered, occasionally damp | Roof structure behind a closed covering |
| 3 | Exterior, above ground, not permanently wet | Cladding, window frames, decking, fencing |
| 4 | Permanent contact with ground or fresh water | Posts in the ground, bank revetment |
| 5 | Permanent contact with salt water | Marine piling, dolphins |
Having set the class, you then choose a species with a suitable durability class (1, very durable, to 5, not durable) — or you have the timber treated.
Applying it in practice
Decide where the timber will sit first, then choose the species. Rules of thumb for DIY work:
- Ground contact is the hardest duty. Untreated softwood put straight into the soil is gone within a few years. Class 4 calls for preservative-treated or naturally durable timber — better still, stand the post on a concrete base, which takes you from class 4 down to class 3.
- Design it to stay dry. Drip edges, an overhang, ventilation behind cladding and the ability to dry out quickly do more for service life than any repainting.
- Every cut matters. In treated timber the protection sits only in the outer zone. Treat every sawn end and every drilled hole with a suitable end-grain preservative.
Risk classes elsewhere
“Risk class” also appears in construction in other senses — for instance grading structures by the consequences of failure (consequence class), or classifying work with hazardous substances such as asbestos. Always check which context the term is being used in.
Related terms
- Durability class
- Exterior durability
- Impregnating
- Facade cladding
- Window frame
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.