When we use the term “durability” it can mean different things, to different people, in different contexts. For example, in the flooring industry, a hard-wearing floor covering may be described as being “highly durable” and thus suitable for use in areas of high foot traffic. Of course there is, within that use of the word “durable”, the concept of “long lasting” – but more associated with its resistance to wear and tear. With timber, there is a similar notion: but we would equate that with surface hardness and/or abrasion resistance. So what do we mean when we say that a particular type of wood has “good durability”?
In fact, we should extend that term and refer to it as “Natural Durability” – because it relates directly to the timber’s own inherent ability to resist attack by biological agencies; most especially decay (rotting fungi). However, the natural durability of any timber species is not something that can be accurately guessed at, simply by looking at the wood: it has to be established by testing samples of the particular timber under controlled conditions of exposure to decay fungi and then seeing how quickly (or how slowly) it may rot away.
Furthermore, the concept of durability in wood relates not to the entire tree, but only to the older, central portion of the tree trunk or log – which is known as its “Heartwood”. The younger, outer band of wood tissue within a tree trunk or log, that is – or was, until the tree was felled – actively taking part on the growing life of the tree, is called its “Sapwood”. All trees that have grown beyond a certain age (and the precise age will vary, depending upon climate, geographical location and forest type) will close down the inner part of the trunk and convert it to “heartwood”, whilst the outer part remains active as “sapwood”. This process continues as long as the tree is growing and expanding: so that the heartwood zone gets bigger, whilst the sapwood zone remains at much the same width throughout the tree’s adult life.
Bearing in mind that it is only the heartwood which may have any degree of natural durability, it is possible – by looking up the wood species in reference books or on reliable websites – to find out the degree to which any timber may resist decay. From this, it can be seen that a timber with good natural durability may be used with greater confidence in more “hazardous” situations, where a less durable species would need to be impregnated with chemical preservatives, in order to prolong its service life. But please remember – it is not simply the colour of the heartwood that shows whether or not a timber is durable: although, as a general rule, timbers with pale-coloured heartwoods (ie, the same colour as the sapwood) will have low natural durability. However, it does not therefore follow that the darker a timber is, the better its durability is (remember the laboratory tests!)
The natural durability of the heartwood of many of the commercial timbers that we commonly use, is rated against an established and agreed scale: with the most long-lasting (ie, decay-resistant) timbers being classified as “Very Durable”; and the next best as “Durable”; followed down the scale by “Moderately Durable”, “Slightly Durable” and lastly, “Not Durable”. Whilst there are no exact figures as to how long such timber may last in service – it being very dependent upon exposure to hazardous conditions and the nature of such hazards (severity of rot, type of soil, submerged fully in fresh water or sea-water, etc) – as a “ballpark” figure, we can estimate with reasonable certainty that a fence post made from a Very Durable wood species will last a minimum of 25 years without preservative treatment (probably a lot longer); whereas one made from a species rated as Not Durable will be lucky to last beyond 5 years without failure. And remember – it is only the heartwood of the timbers that we are talking about: all sapwood of all wood species is rated as “Not Durable”; and so timber containing sapwood will need preservative treatment – or the sapwood will have to be cut away from the timber, leaving only the heartwood, prior to that timber being used.