Perhaps the most important consideration in choosing the lumber species to use for exterior decking is the durability characteristics of the lumber to be used. Unfortunately this is not a precise science as wood is a natural species, with significant variations even within the same species. For example a species grown in one country may have significantly different durability characteristics to the same species grown in another country. This may be due to more rapid growth, harvesting at an earlier age or to a lesser extent, differences in processing. Furthermore, similar species may have sub-species which could exhibit totally different durability characteristics.
Due care should be taken that any wood does not contain the outside sapwood. Sapwood of most hardwood species is non durable and will decay quite rapidly even if the heartwood itself is rated as highly durable. With some cypress species in particular, the sapwood is prone to decay extremely quickly. Fortunately sapwood is generally a lighter color, so can be reasonably easily distinguished.
To a large extent, the denser, harder lumber species generally possess higher durability properties. These are often referred to as Class 1 or “Highly durable". Examples of such species includes Ipe, Cumaru, Tallowwood and Ironbark. Under normal conditions, not in permanent contact with water, such species can be expected to resist decay and insect attack for at least 25 years and up to 50 years.
The next group is Class 2 where the lumber species are termed “Durable". Lumber species included in this group also exhibit exceptional durability characteristics and the wood can typically be expected to have life of 15 to 25 years or more. Some species in this group closely approach the performance of the Class 1 timbers even under severe conditions of service and in fact all species in this group may be regarded as approximating the service of Class 1 timbers where conditions are less severe, as in typical decking. This group includes a large range of species including Teak, Jarrah, Jatoba, Bongossi, Purpleheart, Selangan batu, Western Red Cedar, Merbau, Blackbutt, Spotted Gum, River Red Gum etc.
The third group, Class 3 is termed “Moderately Durable". Such species will give good service without preservative treatment if clear of the ground and used in conditions of low termite hazard and involving only intermittent wetting followed by reasonably rapid drying. Such timbers can still be used for exterior decks but it would probably not be advisable to use such species in severe weather conditions unless a maintenance regime involving the application of good quality decking oil is carried out at regular intervals. Species in this group include Cambara, Kempas, Karri, Eucalyptus saligna.
The final group Class 4 is termed “Non Durable". Timber species in this group are considered unsuitable for exposed decking because of their low natural durability.
But apart from resistance to the ravages of sunlight, rainfall and humidity, in many locations there is another factor to consider – termites. There is no direct correlation between durability and resistance to termite attack so care needs to be taken in termite prone areas. Suitable species with high resistance to termite attack would include Ipe, Cumaru, Jatoba, Teak, Jarrah, Selangan batu etc.
Finally and perhaps of lesser importance is that the lumber chosen should not twist, cup or deform in service. This requires some skill and experience on the part of the lumber mill as the majority of hardwoods require carefully controlled kiln drying to bring the moisture content of the wood down from more than 25% with freshly cut timber to approximately 16%. If the timber is dried to fast it can crack and twist when in service. Even when properly kiln dried, all timber will expand and contract to some extent in service particularly in damp conditions. In such conditions, a timber species with a low shrinkage rate may be preferable. Such species would include Ipe, Selangan batu, Merbau and Teak amongst others.
So in selecting the most appropriate species of lumber for decking, durability is a critical factor but it’s not such a clear cut decision as may be assumed at first glance.
About the author: Malcolm Kay is the CEO of Intex Pacific Pty. Ltd. an international supplier of landscape materials including modular decking tiles. For more information see http://www.swiftdeck.com .