Selecting timber that is fit for purpose is important. We’ll help you understand what to look for so that you’ll have a better idea of what timber to select when specifying a product.
Durability assesses the natural ability of timber to resist rot and fungi decay. That is, how the timber will perform in above ground and in-ground applications without any treatment.
Durability identifies a timber’s ability to stay structurally sound under attack from mould and fungi. It doesn’t mean that the timber will look as good as new in 40 years’ time (the timber will age, shrink, crack, and twist if it’s not protected), but structurally it will remain sound.
Common timber species and their Class rating:
Timber Species | In Ground | Above Ground |
Cumaru | 1 | 1 |
Ironbark | 1 | 1 |
Spotted Gum | 2 | 1 |
Blackbutt | 2 | 1 |
Merbau | 3 | 1 |
Tasmanian Oak | 3-4 | 3-4 |
Clear Pine | 4 | 4 |
Meranti | 4 | 4 |
As a natural product, timber has variations - no two pieces of timber look alike. Characteristics such as knots, gum vein, black speck, fiddleback, borers, or innocent marks and blackheart are all natural features of timber.
Knots | Gum Vein | Blackspeck | Fiddleback | Borers | Blackheart |
The appearance grading of timber addresses the acceptable amount of characteristics:
Select Grade | Medium Grade | High Feature |
Hardness is determined by the Janka rating of the timber. Understanding the hardness of timber is important as the harder the timber, the harder wearing it is and its ability to resist denting and wear.
Timber Species | Janka (kN) |
Cumaru | 14.8 |
Ironbark | 15 |
Spotted Gum | 11.0 |
Blackbutt | 9.1 |
Merbau | 8.6 |
Tasmanian Oak | 5.5 |
Clear Pine | 3.3 |
Meranti | 2.3 |
The Janka rating is determined by forcing a stainless steel ball halfway into the timber and measuring the kilo Newtons force it took to force the ball to that level.
Stability in a timber refers to its expansion, contraction and shrinkage. Timber stability ensures a long lasting design.
Internally, timber should contain 10-12% moisture content and 15-18% externally. At these levels, timber is less likely to expand and contract in a normal environment.
However, it’s very important to understand that environments change. In particular, wall paneling / lining boards and flooring is subject to temperature fluctuations (heating in winter and cooling in summer). Therefore, there is a tendency for the timber to expand and contract in these conditions. By combining a stable timber and a good design during the specification stage you can overcome these stability issues.
Ventilation and drainage are particularly important when using timber externally, especially in cladding and decking. Ensure you have adequate airflow around the timber to prevent it from becoming saturated and staying wet.
End-grain sealers are not commonly used in Australia and they should be. Timber tends to crack on the ends when it is not sealed. Proprietary sealers, such Teknoseal 4000, stop water from moving in and out of the timber preventing the ends from cracking.
When using timber for internal projects, consider its application. In high traffic areas where wear and tear is prominent, use a strong and durable timber species such as Tasmanian Oak. For light traffic areas or ceiling features, use a lighter, low density species such as Clear Pine or Primed FJ Pine.
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