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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. |
