Essential Timber Knowledge

Timber Durability Classes

Introduction to Timber Durability Classes

When selecting a timber to use on a project, great consideration has got to be given to its expected lifespan.  The customary method to determine this is to see where the species sits on the timber durability classification system, as defined by TRADA (The Timber Research and Development Association).  This is a five-tier system that provides a rough guide for how long timber will last immersed in soil, by the ability of the timber’s heartwood to resist decay and infestation. This system is called timber durability classes.

Because the testing is carried out when the timber is immersed in soil where decaying processes are at their destructive best, it is crucial to understand that most timber lifespans will be much longer than those shown in the table below.  For example, a class 1 timber located on a sheltered roof garden will last 150+ years but the same timber in a marine environment will likely last around 20-25 years.

As a rule of thumb timber less than class 3 should not be specified for external use.

Classification System

Durability ClassDesignation Timber Life
(worst case)
1Very Durable25+ YearsTropical Hardwood, Teak, Opepe
2Durable15 – 25 YearsUK Grown Oak, Ekki, Iroko, Other Topical Hardwoods
3Moderately Durable10 – 15 YearsDouglas Fir, Walnut
4Slightly Durable5 – 10 YearsEuropean Elm, Scandinavian Pine, Larch
5Not Durable0 – 5 YearsBeech, Ash, Balsa

For a full list what the class of each timber species is, please follow this link.

Heartwood vs Sapwood in relation to Timber Durability Classes

These classes just refer to the heartwood of timber.  Heartwood is the innermost part of any trunk and forms the majority of a trunks cross section.  Sapwood is the smaller, outer part of the trunk which transports sap up and down the trunk.  Young trees are entirely comprised of sapwood.  You can always spot the heartwood and sapwood in a trunk because heartwood is always a darker colour.

The reason behind this is that the heartwood is dead wood because no substances pass through it.  When it is dying, a chemical is released that makes it stronger and more resistant to insect attacks (also changing the colour).  Therefore, the heartwood provides the structural strength for the tree which is why it is highly valued when compared to sapwood.

In terms of durability, all sapwood whether it be Tropical Hardwood or European Ash is rated as a class 5 timber (not durable).

How are the classes measured?

Classes are measured in relatively harsh conditions.  Typically, a test would involve driving an unprotected, untreated timber pole or stake with a 50mm x 50mm cross section into the ground and monitoring it over time.  Obviously, the stake would be heartwood.

The decay is monitored by timber experts who can accurately assign it a durability class based on their findings.

Therefore, this classification only applies to timber in outdoor environments.  In indoor environments, many of the factors which cause decay are removed so even class 4 and 5 timbers can last many years.  This nicely introduces the next issue to examine, the use-class of the timber.

How does the Use-Class affect Timber Durability and Lifespan?

The use-class refers to the environment where a timber is used.  It is important to consider the use-class because it has a massive impact on the expected lifespan of the timber.  Therefore, the expected lifespan of timber is a tricky question but companies who have experience using timber will be able to offer a good guide.

Are Classes and Sustainability Related?

Class 1 and 2 timbers tend to be less sustainable.  There are several reasons behind this.

Firstly, although class 1 and 2 timbers typically have a longer lifespan, they can take up to 150 years to fully grow which is far longer than other timber classes.  This offsets most benefits gained by the longer lifespan.

Secondly, most class 1 and 2 timbers are grown abroad in tropical regions, like Indonesia or the Congo Basin.  Although this only really applies to UK and Europe, it does lower the attractiveness of class 1 and 2 timbers from an environmental point of view because of the high carbon emissions needed to transport the timber.

Cost Considerations

Timber in a lower class (3,4,5) are generally faster growing and therefore much more widely available, especially in Europe which is where much of these timber species originates from.  Therefore, lower class timbers are less costly.  Also, many Class 1 and 2 tropical hardwood forests are not FSC certified which further drives the demand, and therefore the cost of FSC certified tropical hardwoods.


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