Timber treatment isn’t just a nice-to-have—it’s what helps wood stand the test of time. Whether you’re planning a loft conversion, redoing the floorboards or cladding a garden room, choosing the right treatment grade matters more than most people realise. Left unprotected, timber can become the perfect host for rot, mould and insect damage.
That’s where the timber treatment grades, or “Use Classes”, come in. These classifications are a way of matching the wood’s treatment level to the environment it’ll face—whether that’s a dry living room wall or a fence post set straight into the ground.
There are five Use Classes in total. Each one links to how exposed the timber will be, how much moisture it might take on, and how long it needs to last. Getting this right at the start saves money, hassle, and unexpected repairs later.
Use Class 1 – Internal Dry Applications
For wood that’ll live its life indoors, well away from damp or extreme temperatures, Use Class 1 usually does the job. This grade covers timber used in areas that stay dry—think living rooms, bedrooms or converted lofts where the risk of moisture is basically zero.
It’s not about adding heavy-duty preservatives here. Instead, the treatment is just enough to hold off everyday wear and tear. The focus is on making sure the timber stays clean, stable and free from pests—without overdoing it.
Common Use Class 1 locations include:
- Internal wall studs in dry rooms
- Floor joists above ventilated voids
- Ceiling joists and rafters in insulated, dry loft spaces
- Furniture frameworks and shelving supports
If the space stays dry and sealed, this grade tends to be the go-to. That said, once moisture becomes a possibility—even just condensation—it’s time to look up a class.
Use Class 2 – Internal Damp Applications
Sometimes timber has to cope with a bit more than just room temperature air. Use Class 2 steps things up slightly, covering areas where moisture might appear now and then—usually from condensation, steam or the odd splash.
Kitchens, utility rooms, bathrooms and unheated porches are all common spots for this grade. The timber in these places isn’t getting soaked day in, day out—but it still needs a barrier of protection to stop problems creeping in over time.
Timber treated to this level is usually sealed or coated just enough to resist softening or warping from repeated exposure to indoor humidity. So, if you’re lining a bathroom wall or building units near a combi boiler, this is probably where your timber should sit.
The key thing with Use Class 2? It buys you peace of mind. It’s that extra bit of defence for the parts of your home that might not feel damp but still don’t stay completely dry.
Use Class 3 – External Above Ground Applications
Outside timber doesn’t all get the same treatment. If it’s exposed to the weather but not sitting in soil or water, it typically falls under Use Class 3. This grade covers timber that’s used above ground—out in the open but never buried or constantly soaked.
This kind of treatment helps timber stand up to rain, wind and the odd spell of frost. It also gives it a bit more staying power, especially in parts of the UK where the weather flips from damp to dry and back again more often than we’d like.
Use Class 3 timber is commonly used for:
- External cladding and facades
- Fence rails and panels (not posts)
- Garden furniture
- Pergolas, trellises, and balustrades
- Shed frames and outdoor play structures
It’s often pressure-treated or surface-coated with preservatives that seep into the top layers of the timber. The aim here is to slow down rot and surface decay, but it’s worth saying—once timber touches the ground or sits in pooled water, you’re into a different grade altogether.
Use Class 4 – External Ground Contact Applications
Once timber touches the ground—or sits somewhere that stays wet for long stretches—it needs a much tougher treatment. Use Class 4 is built for that exact situation. This grade covers timber that’s buried, in contact with soil, or exposed to splashing water where drying out properly isn’t likely.
This isn’t just about long-term rot. Timber in these spots can become unstable or even dangerous if it softens or weakens over time. So, it gets a more intense pressure treatment, forcing preservatives deep into the grain. It’s the kind of wood you’d expect to last, even if you forget it’s there for years.
You’ll usually need Use Class 4 timber for:
- Fence posts
- Decking joists and framework
- Raised beds and timber sleepers
- Retaining walls
- Gate posts and signage poles
How to Tell if You Need Use Class 4 Timber:
- Will it be touching the ground? If yes, this grade is almost always required.
- Will it hold weight? For anything structural (like a deck or post), Class 4 gives longer lifespan.
- Will water collect around it? If timber might sit in puddles or soggy areas, treat it as ground contact.
- Are you building something permanent? Long-term projects like fences or retaining walls need the durability of deeper treatment.
Use Class 4 tends to cost a bit more upfront, but skipping it often means redoing the job sooner than you’d like.
Use Class 5 – Marine and Freshwater Contact Applications
Some timber never gets a break from moisture. Piers, pontoons, jetty structures—these live underwater or right on the edge of rivers, lakes, and coastal areas. That’s where Use Class 5 comes in.
This grade is the highest level of timber treatment and it’s not something you’ll usually find in a regular home improvement store. It’s made for timber that stays wet, is splashed constantly, or sits in tidal zones where the waterline moves up and down.
The treatment used here pushes preservatives deeper into the timber than other grades, protecting it against fungal decay, marine borers, and general wear from being submerged for long periods.
Use Class 5 timber is used for:
- Sea defences
- Boat pontoons
- Mooring posts
- Freshwater jetties and dock edges
Timber Durability in Wet Environments
| Use Class | Typical Moisture Exposure | Suitable For | Expected Lifespan* |
| Class 3 | Rain/splash, dries between exposure | Cladding, rails, pergolas | 10–15 years |
| Class 4 | Regular wetting or ground contact | Decks, posts, raised beds | 15–25 years |
| Class 5 | Constant or submerged water exposure | Pontoons, jetties, sea defences | 25+ years |
*Lifespan estimates vary depending on installation, maintenance, and environment.
In short, Class 5 is a specialist choice—but if your timber project sits by the water’s edge, it’s worth asking for by name.
Choosing the right grade of timber treatment isn’t about ticking boxes—it’s what helps your materials last, stay safe, and hold their shape where they’re used. Whether you’re fixing floor joists or putting up a new fence, knowing the right Use Class gives you confidence that the job’s been done properly.
Still not sure which grade fits your project? Feel free to contact us for advice. We’re always happy to help you get it right the first time.