Timber Durability: Choosing timber for a lifetime

The natural durability of timber refers to how well the heartwood of a particular species performs in an exposed situation (outside in the elements). In other words, how naturally resistant a tree species is (without chemical treatment) to attack and destruction by termites, rot etc.

Determining timber durability involves extensive testing and all commercially available construction timbers in Australia have been classified on a Durability Scale of 1-4. This is further divided into Above Ground (AG) and In-Ground (IG) ratings:

Durability Class 1 – Highly Durable:
Timber of the highest natural durability, expected to have a life greater than 25 years in the ground and greater than 40 years exposed above ground.

Durability Class 2 – Durable:
Timber of high natural durability, expected to have a life of about 15-25 years in the ground and 15-40 years exposed above ground.

Durability Class 3 – Moderately Durable:
Timber of moderate natural durability, expected to have a life of about 5-15 years in the ground and 7-15 years exposed above ground.

Durability Class 4 – Not Durable:
Timber of low durability, expected to have a life of 0-5 years in the ground and 0-7 years exposed above ground.

Remember that the above refers to the Heartwood of a species only! All Sapwood is regarded to be Class 4 (non-durable). This is important to remember (see articles on Timber Biology and Timber Treatment for more info on this).

Many species will have different ratings for IG and AG applications. For example, Spotted Gum is Class 1 AG but Class 2 IG, whereas something like Ironbark is Class 1 for both IG and AG applications.

So, if you need a structural post to go into the ground and want a 25+ year lifespan (because it’s holding up your house), you should be asking for a Durability Class 1 IG species like Ironbark. Whereas if you need above ground bearers and joists with a similar life span, Durability Class 1 or 2 AG species like Spotted Gum would be fine.

“But wait,” I hear you say. “Just get treated timber and none of that matters.”

Well, yes and no. The natural durability of timber can certainly be enhanced by chemical treatment (see our blog post on timber treatment), but you need to understand the limitations..

Want to be sure you’re choosing timber that will stand the test of time for your next project? Gowan Lea can provide you with the best timber and lend expert knowledge, ensuring you have an incredible end-product. Call (07) 5448 8711 or visit https://www.gowanleatimbers.com.au/