What are the Durability Classes of Wood?
The Wood Guide · Calculating with wood
The yardstickDurability classes of wood
One number that tells you whether a wood species can go outdoors without treatment. If you remember one thing from The Wood Guide, make it this.
- European standard EN 350
- Five classes
- From 25+ years to less than 5
What the classes mean
The classification is set out in the European standard EN 350 and shows how well a wood species resists fungi, insects and other biological attack on its own.
Important to know: the class tells you about the HEARTWOOD, not the whole board. In almost every species the light sapwood around it is a good deal less durable — with pine and sweet chestnut it is even class 5, while the heartwood reaches class 3 or 2.
And it is not a quality mark. It is a classification of a natural property, measured under standard conditions. What your structure actually achieves depends at least as much on the design: wood that can dry out and does not stand in water beats any coating.
For something that stands unsheltered outdoors all year round: class 1 or 2. Class 3 works provided you treat it regularly. Classes 4 and 5 belong indoors, or must be pressure-treated.
- Standard
- EN 350 (European)
- Measures
- natural resistance to rot and insects
- Applies to
- heartwood, not sapwood
- Class 1
- more than 25 years
- Class 5
- less than 5 years
- Not a
- quality mark or certificate
To be fair: a class is an average under test conditions. Two boards of the same species can differ by years, depending on growth rate, origin and how much sapwood they contain.
Choose a species
The class applies to the heartwood; the lighter sapwood around it is less durable in almost every species. EN 350 is an average under test conditions, not a quality mark. Two boards of the same species can differ by years.
The five durability classes
With, for each class, the species you will find in our register.
| Class | Description | Service life outdoors | Examples |
|---|---|---|---|
| Class 1 | Very durable | more than 25 years | Teak, azobé, iroko, bangkirai, robinia |
| Class 2 | Durable | 15–25 years | Oak, sweet chestnut, western red cedar |
| Class 3 | Moderately durable | 10–15 years | Douglas fir, larch, pine heartwood |
| Class 4 | Slightly durable | 5–10 years | Spruce |
| Class 5 | Not durable | less than 5 years | Beech, poplar, birch |
Which class do you need?
Classes 1 and 2 — the champions
Decking, cladding, hydraulic engineering and garden furniture that stands outdoors all year round. Fifteen to twenty-five years or more, without you having to do anything.
Class 3 — the middle of the pack
Sheltered outdoor uses and structural timber. Ten to fifteen years when used correctly, and an affordable alternative with good performance. Douglas fir is the textbook example.
Classes 4 and 5 — the indoor players
Furniture and interior finishing. Outdoors only if pressure-treated or well protected, and even then with regular maintenance.
Pressure treatment makes spruce (class 4) suitable for use class 3 to 4. That is exactly what makes affordable picnic tables possible that stand outdoors for ten to fifteen years. More on this in the guide to pressure-treated wood.
Design matters more than the class
Class 1 wood that is badly placed lasts less long than class 3 wood that is well placed. That is not theory but everyday practice.
- No ground contactConcrete pad footings or steel anchors. Wood that stands in the soil soaks up water and rots from below — whatever its class.
- Let water run offA slope and no spots where water can pool. Standing water is the main cause of wood rot.
- Let it dryAir circulation around and under the wood does more for its service life than any treatment.
- Protect the end grainThat is where wood soaks up the most. A capping board or an extra coat on the end grain adds years.
Frequently asked questions about durability classes
Why is class 1 best suited to outdoor use?
Can class 4 wood species be used outdoors?
Yes, but only with the right treatment. Pressure treatment lifts them to class 3 to 4, and then they last ten to fifteen years. Untreated, they are finished outdoors within a few years.
What determines the service life of wood?
Three things: the wood species, the treatment and the exposure. The last is the one most often underestimated — wood that can dry out and does not stand in water lasts years longer than the same board in a bad spot.
Does the class apply to the whole board?
No, to the heartwood. The lighter sapwood around it is much less durable in almost every species — with pine it is class 5, while the heartwood reaches class 3 to 4. For outdoor work, ask specifically for heartwood.
Is a durability class a quality mark?
No. It is a classification of a natural property according to a European standard, not a certificate and no guarantee about how the forest is managed. Those are two different questions.
Choose with knowledge
Once you know which class you need, the register points you to the species.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Robinia pseudoacacia — Kenraiz, CC BY-SA 3.0 · Wood test pieces on a fungal culture in a flask (EN 113 test) — G-Lignum, CC BY-SA 4.0