Timber Durability Classes

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What Are Timber Durability Classes?

Timber durability classes are indicators of the suitability of different timbers for different applications. The durability class given to each timber reflects the ability of the heartwood of a given species to resist decay and infestation by insects, such as termites or lyctid borers. The timber durability class of a given species gives a good estimate of how long the timber will last, although it does not take into account all variables which will also effect the lifespan. These variables include, but are not limited to, moisture, temperature, weather, competency of installation, and physical stress exerted upon the timber. It is also important to note that timber durability classes relate only to the heartwood of any species and not the sapwood which is considered class 5, non-durable, for all species of timber.

The Different Classes Of Timber Durability

TRADA, the Timber Research And Development Association, defines five classes of timber durability with 1 being the most durable – ‘very durable’ – and 5 being the least durable – ‘not durable’ – with each having a life expectancy as outlined below:

timber durability classes

Once again, it is important to reiterate that these are guidelines as the conditions the timber is kept in will affect these lifespans (both favourably and adversely), but timbers rated as more durable will last longer in general which is why we choose to use them.

How Are Timber Durability Classes Measured?

The standard test to determine timber durability classes is to take a pole or stake of the heartwood of a certain species of timber, then drive it into the ground and monitor it. Along with the experience of timber experts, the results of the timber’s deterioration will be used to assign a durability class. In protected conditions, such as indoors with no insect infestation, most hardwoods have a lifespan of more than 50 years which is much longer than when unprotected.

Heartwood vs. Sapwood

You might be wondering what is heartwood and what is sapwood? Sapwood is the outermost, living part of any trunk or branch, whilst heartwood is the innermost, dead part of any trunk or branch and comprises the majority of any stem’s cross section. It is normally possible to distinguish the heartwood from the sapwood by the colour – heartwood is darker than sapwood, as you can see in the picture. When it comes to durability, the sapwood of all species is rated as a class 5, meaning not durable.

sapwood heartwood comparison

The dark heartwood is surrounded by lighter sapwood.

In a young tree, only sapwood is present, but as it becomes older, the centre of the tree dies to form heartwood. As the wood dies, a chemical is released that changes the colour of the wood whilst making it stronger and more resistant to insect attacks. This is vital for the tree because heartwood provides the structural strength for the tree to stand whilst sapwood transports sap around the tree. The ratio of heartwood to sapwood depends on the number of leaves a tree has and how fast it grows. More leaves and quicker growth require more water, hence a higher abundance of sapwood. The image above shows the cross sections of a maple (large leaves) on the left and a black locust tree (small leaves) on the right, hence the difference in heartwood to sapwood ratios.

Durability Of Different Timbers

The following graphic shows the durability classes of 149 different timbers – 116 hardwoods, 29 softwoods, and 4 modified timbers.

timber durability chart

Image Credit: Wood Workers UK

Why We Use British Oak

As you can see from the chart, British oak is classified under durability class 2, meaning durable. Class 2 timbers are suitable for outdoor use and will withstand the elements for many years, meaning that our structures are long lasting. Oak is readily available in the UK from responsibly managed sources that are FSC certified or equivalent, meaning we can source a durable timber locally, reducing any negative impact upon the environment. By comparison, class 1 timbers are mostly grown outside the UK and have often been logged aggressively, causing several species to become endangered, which is against our policy to be environmentally sustainable. Anything less than class 2, ‘durable’, would not offer the structure lifespan that we want to offer to our customers, so we avoid using any of these timbers as well. Find out more about our British oak supply here.

Buying British Oak

If you are looking to buy British oak, please check out our oak supply page. We opened an oak sawmill in 2017 to cope with our high level of demand for British oak and now supply clients all over the UK under the name Quercast Sawmilling. You can visit the Quercast Sawmilling website here to find out more about our British oak supply.

Is That Everything?

If you have any further questions about timber durability classes, heartwood and sapwood, or anything else, please don’t hesitate to get in contact with us.