Durability is a measurement of how long specific species of timber would last if a 50mm x 50mm cross section was left in the ground unprotected. An untreated Oak beam, which is classed as ‘durable’, for example, will last 15-25 years depending on environmental conditions. The following scale of durability was outlined by TRADA, the Timber Research and Development Association.
| Durability |
Class designation |
Average service life |
| 1 |
Very durable |
More than 25 years |
| 2 |
Durable |
15-25 years |
| 3 |
Moderately durable |
10-15 years |
| 4 |
Slightly durable |
5-10 years |
| 5 |
Not durable |
Less than 5 years |