What Is Timber Durability?

In order to determine the suitability of different timbers for given applications, we use a rating called timber durability. Each timber is assigned a class from one to five which reflects the ability of that timber’s heartwood to resist decay and infestation by insects. In other words, how long will a timber last? It does not consider the lifespan of the sapwood for any species as all sapwood is rated as a class five, meaning not durable. Durability classes cannot take into account local conditions such as temperature, moisture, weather, physical stress upon timber, and competency of installation, but they do give a good guide as to how long any type of timber will last.

Timber Durability Classes

There are five different classes of timber durability, as defined by TRADA, The Timber Research and Development Association, ranging from class five to class one, where class five is consider not durable and class one is considered very durable. The life expectancy and designation of each class is outlined in the table below:

timber durability classes

Due to local conditions affecting a timber’s lifespan favourably or adversely, the above table is to be used as a guideline. However, timbers rated as more durable will last longer than less durable timbers which is why we choose to use them. Both oak and sweet chestnut, the two timbers we use most in our creations, are rated as class two, which means durable, and have a life expectancy of fifteen to twenty-five years.

How To Measure Timber Durability

A typical test to determine a timber’s durability is to drive a pole or stake of heartwood into the ground and monitor it over time. When combined with the knowledge of timber experts, the timber can then be assigned to a durability class. This indicates the level of durability in outdoor environments; in protected conditions, such as indoors without risk of insect infestation, most hardwoods will last more than fifty years.

The Difference Between Heartwood And Sapwood

Within the stem, or trunk, of any tree, you can make two loose divisions; heartwood and sapwood. Heartwood is the innermost, dead part of any stem which provides structural support for the tree whilst the sapwood is the outermost, living part of any stem that transports sap up and down the tree. As a tree matures, the heartwood to sapwood ratio generally increases whereas very young trees will consist entirely of sapwood. If you look at the cross section of any stem, you can normally tell the difference between the heartwood and sapwood by the colour; sapwood is much lighter than heartwood, as you can see in the picture below. All sapwood is rated as a class five, meaning not durable, which is why we use only the heartwood in our constructions.

sapwood heartwood comparison
The living sapwood is lighter in colour and surrounds the dead heartwood

Heartwood is created by a chemical that is released as cells die. This chemical makes the dead heartwood of a tree become stronger and more resilient to insect attacks which is vital for the stability and longevity of any tree. In addition to age, the ratio of heartwood to sapwood is also dependent upon the speed a tree grows and the number of leaves it has. Faster growth and more leaves require more water, hence a greater abundance of sapwood. The cross sections above show a maple tree, which has large leaves, on the left and a black locust tree, which has small leaves, on the right, clearly showing the difference in sapwood to heartwood ratios.

Timber Durability Chart

Featuring 116 hardwood, 29 softwoods, and 4 modified timbers, the following chart gives the durability of 149 different timbers.

timber durability chart
Image Credit: Wood Workers UK

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

Why We Use British Oak And Sweet Chestnut

We use a variety of timbers depending upon the project in question, but the two timbers we use more than any other are UK grown oak and sweet chestnut. Both oak and sweet chestnut are classified as class two timbers, designated as durable. Durable timbers are suitable for outdoor use, where most of our creations are located, and can be expected to have a lifespan of fifteen to twenty-five years. We believe in making long lasting structures that will stand up to the British weather without the use of harmful preservatives; using durable timbers allows us to achieve this. Sourcing local timbers from FSC managed forest allows us to minimise our impact upon the environment, preserving a better world for future generations.

A UK Oak Supplier

We opened Quercast Sawmilling in 2017, a supplier of UK grown oak, to satisfy our own needs, but we now offer an oak supply and sawmill service, supplying clients all over the UK. You can find out more about us as an oak supplier here or visit the Quercast Sawmilling website. We can also supply sweet chestnut and other hardwood timbers.

Is There Anything Else You Would Like To Know?

We have decades of experience working with different timber, so if you have any other questions about timber durability, please don’t hesitate to get in contact with us.