Quick answer: UC3 is normally used for exterior timber above ground. UC4 is used where timber is in permanent ground or fresh-water contact and for exterior structural support. Choose the Use Class from the job the timber has to do, not from whether it looks green or brown.
Which treatment class do you need?
| Job | Normal starting point |
|---|---|
| Fence boards and rails above ground | UC3 |
| Exterior cladding | UC3 |
| Decking boards | UC3 |
| Fence posts installed in the ground | UC4 — see our UC4 treated fence posts |
| Deck posts | UC4 |
| Exterior structural joists and beams | UC4 |
| Timber in permanent ground or fresh-water contact | UC4 |
UC3 is for outdoor timber used above ground
The Wood Protection Association describes Use Class 3 as exterior timber used above ground. Typical examples include fence boards, rails, cladding and decking boards where the timber can get wet but is not being used in UC4 exposure.
UC4 is not just for timber buried in the ground
Use Class 4 covers timber in permanent ground or fresh-water contact, but it also covers exterior structural support. That distinction matters.
A fence post set in the ground is an obvious UC4 example, but deck posts, exterior structural joists and beams can also require UC4 even when the whole component is not buried.
Our UC4 Treated Softwood Fence Posts are specified for ground-contact fencing applications.
Do not choose Use Class from colour alone
Two treated timbers can look very similar while having different treatment specifications. Green or brown colour does not prove that timber is UC3 or UC4. Check the product specification.
Good detailing still matters
Correct treatment does not make poor installation irrelevant. Avoid unnecessary water traps and provide drainage and ventilation where the construction requires them.
Re-treat freshly cut or drilled treated timber
Cross-cutting, drilling, boring or notching can expose timber that has not received the same preservative penetration as the original treated surface. Re-treat freshly exposed areas with a suitable cut-end preservative in line with the treatment guidance.
Do not install a freshly cut end into the ground. Plan ground-contact components so the factory-treated end remains the end exposed to the most severe conditions.
For more detail, read Do You Need to Treat Cut Ends of Pressure-Treated Timber?.
If you are unsure, use the higher Use Class
Where the exposure is genuinely uncertain, do not downgrade the treatment specification to save a small amount on the timber. Use the higher appropriate Use Class or check the project specification before ordering.
Choose the product for the job
Browse Treated Timber for outdoor timber products. For fence posts, decking subframes, exterior structural work or any component close to or in the ground, check the individual treatment specification rather than choosing on section size alone.
Source: Wood Protection Association preservative treatment guidance.
