Which wood can withstand moisture?
The Wood Guide · Choosing and comparing
Buying guideWhich wood can withstand moisture?
A bathroom, an outdoor kitchen and a bank revetment are three completely different questions. The answer starts with: exactly how wet does it get?
- Use class 1 to 5
- Wet ≠ wet
- Teak, Accoya, azobé
Which wood can withstand moisture?
A bathroom, an outdoor kitchen and a bank revetment are three completely different questions. The answer starts with: exactly how wet does it get?
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The question behind the question
"Can this wood withstand moisture" is not one question but five, and the standard numbered them long ago.
The use classes describe how harsh the environment is: class 1 is dry indoors, class 2 is indoors with a risk of condensation, class 3 is outdoors without ground contact, class 4 is ground or fresh-water contact and class 5 is permanent salt water.
The durability classes describe how well a wood species resists decay. A species suits an environment if its durability class is high enough for the use class on site. So a bathroom cabinet and a sheet pile wall literally ask for different things.
Wood does not rot from water. It rots from fungi, and they need water to live. Below about twenty per cent moisture content, no wood-rotting fungus can grow. That is why wood that gets wet and dries out again quickly is far less vulnerable than wood that stays damp all the time — even though the former has to cope with more water.
- Use class 1
- dry indoors
- Use class 2
- indoors, risk of condensation
- Use class 3
- outdoors, no ground contact
- Use class 4
- ground or fresh-water contact
- Use class 5
- permanent salt water
- Critical threshold
- below 20% moisture no rot fungus grows
To be fair: no wood species is waterproof in the sense that nothing happens. Teak swells too when it gets wet, and ipe turns grey too. What sets the good species apart is that they do not rot — not that they stay unchanged.
Which wood in which situation
| Use class | Situation | Suitable species |
|---|---|---|
| 1 | Dry indoors | All species; including beech, ash, pine |
| 2 | Indoors with condensation: bathroom, cellar | Cedar, western red cedar, teak, Accoya, moisture-resistant boards |
| 3 | Outdoors, no ground contact: deck, cladding, garden furniture | Teak, ipe, Douglas fir, larch, pressure-treated spruce, thermally modified wood |
| 4 | Ground or fresh-water contact: posts, bank revetment | Robinia, class 4 pressure-treated wood, azobé, bilinga |
| 5 | Permanent salt water: locks, harbour works | Azobé, bilinga |
A common mistake: a bathroom feels wetter than a garden, so people choose garden timber. But a bathroom is warm and dries out in between; a piece of garden furniture stands in the freezing cold and does not dry for days. For a bathroom cabinet, cedar, teak or moisture-resistant board material is more than enough.
The species that tolerate moisture best
Teak
Extremely high natural oil content, used in shipbuilding for centuries. Class 1, weathers beautifully and needs minimal maintenance. The benchmark.
Accoya
Acetylated softwood that achieves class 1 from European wood, and that also barely swells — exactly what you want in a wet environment.
Azobé and bilinga
The only two that can handle permanent salt water. You build lock gates and sheet pile walls with them, not furniture.
Western red cedar
Light, fragrant and naturally resistant to fungi. The classic for saunas, bathroom panelling and cladding.
More important than the wood species is the design. Make sure water runs off and air can get in: gaps between boards, legs off the ground, a cavity behind cladding. Wood that gets wet and dries quickly lasts longer than more durable wood that stays damp.
Frequently asked questions about wood and moisture
Which wood is the most water-resistant?
Which wood can go in the bathroom?
A bathroom is use class 2, less harsh than outdoors. Cedar, teak, Accoya and moisture-resistant board material are more than adequate. Above all, make sure there is good ventilation.
Why does wood actually rot?
Not because of water itself, but because of fungi that need water. Below about twenty per cent moisture content no wood-rotting fungus grows. That is why wood that dries quickly is far less vulnerable.
Can pressure-treated wood go in the ground?
Yes, provided it has been treated to use class 4. That is a heavier treatment than the usual class 3. See pressure-treated wood.
Moisture in perspective
Start with how wet it gets, not with which wood you like the look of.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Kingston by Ferring, West Sussex — Fernweh, CC BY-SA 2.0 · Timber revetment along a garden ditch, in permanent contact with water — Mbbma, CC BY-SA 4.0