This is the fourth in a series of technical articles that aim to inform the reader of the different technical properties of wood and how these properties can affect the behavior of timber in application. This feature will discuss matters concerning durability.

Durability of Timber

The durability of a timber species refers to its ability to resist attack from pests which have the ability to utilise the wood as a food source and, at the same time, have the potential to cause damage - sometimes even serious structural damage - if left unchecked. These pests include wood decaying fungi, which can be present if the timber is exposed to moisture for long periods of time, or termites, which are present in the warmer regions of the world. It is important to note that when we talk about durability, we are only referring to the durability of the heartwood of the timber.

The heartwood is the central part of the tree, which has shut down and plays no further role in conducting sap upwards or in the continued growth of the tree. The growing role is performed by the outer part of the tree - the sapwood. The sapwood, even when dried and after it has been sawn into boards, contains nutrients such as sugars and carbohydrates which fungi and insects are attracted to. The sapwood of all timber species is what wood scientists would refer to as ‘not durable’ as it is a lot more susceptible to the risk of decay or insect attack. Even the sapwood of teak is not durable!

It is the heartwood that contains the waste deposits from the growth of the tree, which are known as extractives and which are much less attractive to insects and fungi than the sapwood. The presence of these extractives will very often change the color of the heartwood, generally making it darker and distinct from the lighter-colored sapwood. It is also these extractives that make the wood resistant to attack to a lesser or greater degree, depending on the species. This will give the timber a level of ‘natural durability’, which is a species’ own ability to withstand attack without any form of added preservative treatment.

Natural durability varies greatly between timber species. An assessment is made in order to determine the difference, which, for resistance to fungal decay, is usually derived from the performance of hardwood stakes exposed half buried out of doors in soil. In Europe, the performance of a wide range of commercial timber species, both softwoods and hardwoods are listed in the European Standard BS EN 350-2, where they are categorised in a five grade Durability classification depending on how long they are able to resist decay in the ground:

Class 1: Very Durable - for example teak, ekki - more than 25 years

Class 2: Durable - for example utile, western red cedar - 15 to 25 years

Class 3: Moderately Durable - for example American walnut, Douglas fir - 10 to 15 years

Class 4: Slightly Durable - for example American red oak, obeche - 5 to 10 years

Class 5: Not Durable - for example ash, beech, all sapwood - less than 5 years

If the heartwood has a high natural durability against fungal decay, then it will generally give the same natural protection against insects, including termites. The most durable timber species that is commercially available from the North American temperate forest is American white oak, which falls between Class 2 and 3. Generally speaking, the most durable timber species in the world grow in tropical forests.

The durability of the timber is an important consideration when deciding where it will be used and what conditions it will be subjected to. The risk of fungal decay all depends on the exposure to moisture. If the timber isn’t going to get wet, then there is no risk of decay and sapwood can be included, such as in interior joinery or furniture. If the risk of exposure to moisture increases and if sapwood is to be included, then it should be adequately treated with a suitable wood preservative. In tropical regions, the risk of wood boring insects is so much greater that, even in dry conditions, the sapwood should be restricted if the timber is untreated for permanent use. In these regions, it is advisable to exclude the sapwood completely for high end applications, such as flooring and furniture.

Although sapwood may be used in a number of circumstances, it must not be overlooked that its presence is a potential source of weakness should the site or building conditions change. This should always be taken in to account when specifying timber for a particular end use, particularly if the intention is for a long service life. In the next Technical Matters we will look at different methods of improving the durability of timber using preservation, as well as modern methods of wood modification.