Which wood species are best for long-term outdoor use?
The Wood Guide · Choosing
ChoosingThe best wood species for outdoors
There is no best wood species — there is a best species for a particular place. Ground contact, decking and cladding all ask for something different.
- Class 1 to 5
- Location determines lifespan
- Modified wood counts too
Durability class in one minute
The European standard divides wood species into five classes, based on how long the heartwood holds out in contact with the ground.
Class 1 means very durable, class 5 not durable. Spruce is in class 4, pine (heartwood) in class 3 to 4 — which is why they must be treated before they can go outdoors. Teak, azobé and robinia are in class 1 and need nothing, not even in the ground.
Important: the class applies to the heartwood. The sapwood — the pale outer band of the trunk — is not durable in almost every species. A board with a lot of sapwood in a class 1 species behaves outdoors like class 5 wherever that sapwood is.
And location matters as much as species. The same board lasts three times less in the ground than above it, and above ground under a roof much longer again than when exposed. Every lifespan you read is an estimate for a particular use.
Durability class is about attack by fungi and insects. It says nothing about greying, cracking or movement. A class 1 species that does not rot can still crack and change colour — that is a matter of finish, not of class.
- Class 1
- very durable — teak, azobé, robinia, afzelia
- Class 2
- durable — oak heartwood, western red cedar, iroko
- Class 3
- moderately durable — larch, Douglas fir
- Class 4 and 5
- not durable — spruce, pine, poplar
- Modified
- Accoya and Kebony reach class 1
- Sapwood
- not durable in almost every species
- Ground contact
- greatly shortens lifespan
To be fair: the most durable species are almost all tropical, heavy and expensive, and their origin deserves attention. The European alternatives — robinia, oak, larch — are less available in large sizes and often less dimensionally stable. No species is cheap, light, stable, durable and local all at once.
Choose a location
The class applies to the heartwood; sapwood is not durable in almost every species. And the same board lasts three times less in the ground than above it.
What the page recommends here
What lasts how long
The lifespans below apply to use above ground, in our climate, without a finish.
| Species | Class | Above ground | Characteristic |
|---|---|---|---|
| Teak | 1 | very long | high oil and resin content; moves little |
| Azobé | 1 | very long | extremely hard and heavy; also in ground contact |
| Robinia | 1 – 2 | long | the European class 1; often grows crooked |
| Afzelia (doussié) | 1 | very long | dimensionally stable, used in windows and doors |
| Iroko | 1 – 2 | long | often used as a teak substitute |
| Oak (heartwood) | 2 | long | tannin stains on iron; sapwood not durable |
| Western red cedar | 2 | long | light in weight, soft, scratches easily |
| Larch | 3 – 4 | average | attractive figure, moves a lot |
| Douglas fir | 3 | average | large sizes available, resin pockets possible |
| Accoya (acetylated) | 1 | very long | modified softwood; expensive |
| Pressure-treated spruce | from 4 to 1 – 2 | long with good impregnation | the affordable standard for garden timber |
Choose by use class, not by species
In the ground
Posts, garden screens, foundations. Only class 1 or properly pressure-treated wood. This is the heaviest load there is.
On the ground, horizontal
Decking boards and platforms. Water lies on them, so class 1 to 2, or thermally modified wood that absorbs little water.
Above ground, exposed
Garden furniture, pergolas, fence panels. Class 1 to 3 works here, and pressure-treated softwood performs excellently.
Above ground, sheltered
Under a roof or canopy. Almost any species goes a long way here, including untreated larch and Douglas fir.
Cladding
Dimensional stability counts for more than durability here: thermally modified wood, western red cedar or acetylated wood.
Fresh or salt water
Bank revetments and jetties call for azobé or an equivalent class 1 species. Anything less just postpones the problem.
Why one species holds out and another doesn't
Natural oils and resins
Teak and iroko contain substances that repel water and deter fungi. That is not a treatment but a property of the wood itself.
Density
The denser the fibre structure, the less water can get in. That is why azobé is almost indestructible — and almost impossible to work.
Extractives
Tannin in oak and similar substances in robinia make the wood unattractive to fungi and insects.
Dimensional stability
Wood that shrinks and swells little gets fewer cracks — and cracks are exactly where water gets in.
You don't necessarily have to look to the tropics. Robinia, oak heartwood and well pressure-treated spruce do what is asked of them in most gardens. Which species from our region hold up outdoors is covered separately.
Frequently asked questions about wood species for outdoors
What does durability class 1 mean?
Very durable: the heartwood resists fungi and insects for a very long time, even in contact with the ground. Class 5 is not durable.
Does the class apply to the whole board?
No, only to the heartwood. The pale sapwood on the outside of the trunk is not durable in almost every species.
Which wood can go in the ground?
Class 1 species such as azobé or robinia, or softwood that has been pressure-treated. Anything below that fails within a few years.
Is tropical hardwood always better?
Not always. For a deck or garden furniture, well pressure-treated spruce performs excellently at a fraction of the price and the weight.
Does a durable wood species keep its colour?
No. Durability class is about rot, not UV. Every untreated species turns grey outdoors, teak included.
What is modified wood?
Softwood that has been treated chemically or thermally so that it gains the durability of tropical hardwood. Accoya and Kebony are the best-known names.
More in The Wood Guide
Each species above is covered separately, with figures, uses and the honest drawback.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Lophira alata · Kastrup Søbad, Copenhagen — RhinoMind, CC BY-SA 3.0 · Robinia pseudoacacia — Famartin, CC BY-SA 4.0