What is a durability class?
A durability class is the classification under the European standard EN 350 that indicates how naturally resistant a timber species is to attack by fungi and wood-boring insects. Timber is divided into five classes, with class 1 being very durable and class 5 not durable. The durability class helps select the right species for the right application.
The five durability classes
| Class | Description | Expected life in ground contact | Examples |
|---|---|---|---|
| 1 | Very durable | > 25 years | Teak, azobé, ipe, bilinga |
| 2 | Durable | 15-25 years | Oak, accoya, black locust |
| 3 | Moderately durable | 10-15 years | Douglas fir, larch, sweet chestnut |
| 4 | Slightly durable | 5-10 years | Spruce, pine (treated) |
| 5 | Not durable | < 5 years | Beech, poplar, alder |
How is the class determined?
The EN 350 standard is based on laboratory and field tests in which timber is exposed to:
- Fungi — brown rot, white rot and soft rot
- Wood-attacking insects — house longhorn beetle, woodworm, termites
- Ground contact — all kinds of organisms in damp soil
- Marine organisms — important for hydraulic engineering
Use class versus durability class
Alongside the durability class is the use class (EN 335). It describes the conditions in which the timber is used:
- UC 1 — dry, internal use
- UC 2 — occasionally damp
- UC 3 — external, not in ground contact
- UC 4 — external, in ground contact
- UC 5 — permanently in salt water
For its use class, a species must have at least a matching durability class.
Application
The durability class determines:
- Façade cladding — class 3 minimum, class 2 preferred
- Window and door frames — class 2 or treated class 3
- Fences and garden timber — class 2 for longer service life
- Decking and terraces — class 1 or treated class 2
- Hydraulic structures — class 1 only
Wood preservation
Timber of a lower durability class can be upgraded to a higher class by treatment (impregnation, acetylation, thermal modification).
Related terms
- Timber species
- Accoya
- Wood rot
- Impregnation
- FSC
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