Timber and Sustainability

A Guide to Timber Durability

What is Timber Durability? 

Timber durability is a metric designed to assess how suitable timber types are for their various uses in architecture and design. 

Each timber is categorised on a scale from one to five, indicating the resistance of its heartwood to decay and insect infestation. Essentially, this rating indicates how long a particular timber is likely to endure.  
It's important to note that this assessment doesn't factor in the longevity of sapwood for any species, as all sapwood is uniformly rated as class five, and while durability classes don't encompass local environmental factors like temperature, moisture, weather conditions, physical strain on the timber, or the quality of installation, they provide a reliable estimate of the lifespan of various timber types. 

Timber durability

Timber Durability Classes 

TRADA, The Timber Research and Development Association, has established five classes of timber durability, ranging from class one to class five. Class five signifies low durability, while class one indicates very high durability. The table below outlines the life expectancy and designation for each class: 

Given that weather conditions can either enhance or diminish the lifespan of timber, the table provided serves as a general reference. Nevertheless, timbers with higher durability ratings tend to outlast those with lower ratings, hence the general preference for them.  
Among commonly used materials, such as European oak and chestnut, both rated as class two (indicating durability), have an expected lifespan of fifteen to twenty-five years in our creations. 

Timber durability

How is Timber Durability Tested? 

A common method for assessing timber durability involves embedding a heartwood pole or stake into the ground and observing its condition over time. By complementing this test with insights from timber experts, the timber can be categorised into a durability class, reflecting its ability to withstand outdoor conditions. In protected environments, such as indoors without the threat of insect infestation, the majority of hardwoods are expected to endure for over fifty years. 

Timber durability

Heartwood vs Sapwood 

In the trunk or stem of a tree, there are two distinct sections: heartwood and sapwood. Heartwood, the innermost portion, serves as the tree's structural support, while sapwood, the outermost layer, transports sap throughout the tree. As a tree matures, the ratio of heartwood to sapwood typically increases, whereas in very young trees, sapwood dominates. By examining the cross-section of a stem, one can often discern the contrast between heartwood and sapwood by their respective colours as sapwood is noticeably lighter than heartwood. Due to its low durability rating (class five), which renders it vulnerable, constructions exclusively utilise heartwood. 

Contact the team today to discuss our range of architectural timber.