Pressure-treated wood is softwood (usually spruce or pine) that has been treated under high pressure with preservatives to make it resistant to mould, rot, and insects. The wood is placed in an autoclave where the preservative is forced deep into the wood fibres. Recognisable by its greenish colour.
How does pressure treatment work?
1. The wood is placed in a pressure vessel (autoclave)
2. Air is extracted from the wood (vacuum)
3. Preservative is forced into the wood under high pressure
4. The agent penetrates deep into the wood fibres
This is fundamentally different from staining or painting, which only treats the surface.
Lifespan
| Application | Lifespan (indication) |
|---|---|
| In the ground | 10-15 years |
| Outdoors, not in the ground | 15-20 years |
| Under cover | 20-25 years |
| Untreated spruce (for comparison) | 2-5 years outdoors |
When do you use pressure-treated wood?
| Suitable | Not suitable |
|---|---|
| Fence posts and boards | Indoors (health concerns) |
| Garden fencing | Raised vegetable beds (ground contact with food) |
| Pergolas and covered structures | Children’s furniture |
| Play equipment (CE-certified) | Chopping boards / kitchen utensils |
| Deck substructure (joists) | Visible finishes (unattractive green colour) |
Pressure-treated wood in DIY projects
It is the budget option for outdoor timber:
- Building a fence — Posts and boards of pressure-treated wood
- Decking — The substructure (joists, beams) of pressure-treated wood, the deck boards of Douglas fir or hardwood
- Pergola — Structural timber pressure-treated, visible timber in Douglas fir
- Garden shed — Framework and structure pressure-treated, exterior cladding to choice
Working with pressure-treated wood
- Treat cut ends — Where you cut or drill, the wood is unprotected. Brush cut surfaces with wood preservative (e.g. copper-based preservative)
- Pre-drill — Pressure-treated wood is slightly more brittle. Always pre-drill for screw connections
- Do not paint immediately — The wood must dry for 3-6 months before it can be painted. The preservatives inhibit paint adhesion.
- Clean up sawdust — Do not inhale the sawdust. Work outdoors where possible and use a dust mask.
Pressure-treated vs. other treatments
| Treatment | Method | Durability | Appearance |
|---|---|---|---|
| Pressure-treated | Pressure treatment | 15-20 years | Greenish, weathers grey |
| Thermowood | Heat treatment (180-220°C) | 15-25 years | Warm brown |
| Douglas fir (untreated) | Naturally durable | 15-20 years | Weathers silver-grey |
| Oil/stain | Surface treatment | 1-3 years (reapply) | As desired |
Related terms
- Softwood
- Spruce
- Thermowood
- Douglas fir
- Fence post