As wood is a natural product, there can be quite significant variations even within the same lumber species.  For example, there can be differences because of the regions from which the lumber is growing, which will cause it to have different growth characteristics.  And harvesting may be done at an earlier age or there can be differences in processing of the timber.  In addition some species have numerous subspecies where the durability characteristics can be significantly different e.g. red balau compared with yellow balau.

Apart from any normal durability considerations, due care should also be taken that any wood does not contain sapwood. Sapwood of most hardwood species is totally 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 with most lumber species.

"Highly durable" - Class 1

Generally speaking, the denser, harder lumber species possess higher durability characteristics. These are often referred to as Class 1 or “Highly durable”. Examples of such species includes Ipe, Cumaru, Tallowwood and Ironbark.

"Durable" - Class 2

Lumber species 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.

"Moderately durable" - Class 3

Species in this group will give good service without preservative treatment if kept 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. 

"Non Durable" - Class 4

Timber species in this group are considered unsuitable for exposed decking because of their low natural durability.

Termite resistance

Apart from resistance to the ravages of sun rainfall and humidity, in many locations termite resistance is another factor to consider. But there is no direct correlation between durability and resistance to termite attack so care needs to be taken in choosing the right lumber species in termite prone areas. Suitable species with high resistance to termite attack would include Ipe, Cumaru, Jatoba, Teak, Jarrah, Selangan batu etc. 

Twisting, cupping and bowing

Finally and perhaps of lesser importance provided the planks are nailed or screwed securely to the bearers, is that the lumber chosen should not twist, cup or deform in service. Although this factor can be related to the individual species, it also relates to the kiln drying process.  Drying of the timber 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.

Species that are regarded as particularly stable in respect of twisting cupping and bowing would include Ipe, Selangan batu, Merbau and Teak in particular.