Anyone building a fence or setting stakes will sooner or later come across the term durability class. This article explains what the standard DIN EN 350 means by it, how it classifies robinia, oak, larch and spruce, and why the class alone says nothing yet about an individual component.
What does durability class mean?
Under DIN EN 350, the durability class describes the natural resistance of a timber to wood-destroying fungi, without chemical timber preservation. The classification is based on tests and empirical values for the timber species concerned.
The standard considers the heartwood, i.e. the inner, older part of the log. The sapwood in the outer part is generally regarded as not durable. How much sapwood a component contains is therefore just as important as the timber species.
- Heartwood
Inner part of the log, usually denser and naturally protected by extractives. - Sapwood
Outer part of the log, richer in nutrients and therefore more susceptible to fungi.
The five classes under DIN EN 350
The standard distinguishes five classes. The lower the number, the higher the natural resistance to fungi.
| Class | Designation |
|---|---|
| 1 | very durable |
| 2 | durable |
| 3 | moderately durable |
| 4 | slightly durable |
| 5 | not durable |
The class is a classification of the heartwood of a timber species. How a particular component behaves also depends on the location, moisture, soil type, construction, installation and maintenance. For this, the standard DIN EN 335 divides the service conditions into use classes, from dry indoors to permanent ground contact.
How DIN EN 350 classifies the four timber species
In the common classification under DIN EN 350, the four timber species are assigned as follows. The revised version of the standard gives wider ranges for some species, because origin and growth play a part.
| Timber species | Class under DIN EN 350 | Designation |
|---|---|---|
| Robinia | 1 to 2 | very durable to durable |
| Oak | 2 | durable |
| Larch | 3 to 4 | moderately to slightly durable |
| Spruce | 4 | slightly durable |
The classes reflect the classification of the timber species under the standard. They are not a statement of how long a particular component will last.
What matters for timber in ground contact
In the ground, moisture, microorganisms and oxygen come together. Wood-destroying fungi grow particularly well under these conditions. That is why the durability class has a greater effect in ground contact than with components above ground that dry out well.
- Standing water: if moisture stays in the timber for a long time, the risk of rot increases. Water should be able to run off.
- End faces: end grain absorbs water particularly quickly. A chamfer on the stake head or a slight slope helps water to run off.
- Construction: clearance from the ground, ventilation and avoiding waterlogging are part of design detailing for durability.
- Maintenance: remove leaves from the foot of the stakes and cut back vegetation so that the timber can dry out.
The four timber species in detail
Robinia is very dense and contains natural extractives that counteract fungi. DIN EN 350 classifies its heartwood as durable to very durable. This applies above all to the heartwood of older trees. In timber from very young, fast-grown stands, the proportion of juvenile wood can be higher and the natural durability lower.
Oak has a high density and contains tannins that inhibit decay by fungi. Its heartwood is classified in durability class 2.
Larch is resinous and is classed as moderately to slightly durable. Among the native softwoods it is one of the more resistant timber species. Outdoors, larch turns grey; this patina is a natural process.
Spruce is classed as slightly durable. For components in ground contact it is therefore generally used with timber protection, for example pressure-treated.
Choosing the timber species
Which timber species is suitable for a project depends not only on the durability class but also on the installation situation, construction, loading and budget, and has to be decided for the specific project. The differences between cleft and round posts are described in the article Cleft post or round post, and setting stakes in the article Setting robinia stakes.
This article provides general information on the classification of timber species under DIN EN 350. It does not describe any properties of the goods supplied by us and does not warrant any such properties. Only the details in the quotation and in the order confirmation are authoritative.
- DIN EN 350: durability of wood and wood-based products, classes 1 to 5
- DIN EN 335: use classes; ground contact corresponds to use class 4
The standards named are a guide; what applies is always the version in force and assessment on the actual object.
