information/FAQs

Durability - Timber Grades and Size

Australian Standard grading rules usually provide a range of grades that can be selected to suit specific applications, e.g.:

for hardwood sawn and milled products, select, medium feature and high feature grades are described in AS 2796. For softwoods, grades are described in AS 1782-1787 and AS 1492-1498.
for stress graded timber, Structural Grades 1 to 5 are specified with additional appearance grades optional in AS 2082 and AS 2858.

General structural grades may not provide for, or limit, defects that can have a bearing on durability in exposed applications.

Other factors affecting durability that may need to be considered include untreated sapwood, sapwood orientation (sapwood should be placed to the outside of joints or exposed to higher levels of ventilation), open defects, loose knots, voids, splits, etc.

The cross-sectional size and proportions of members should be considered when detailing for durability. This is particularly important for unseasoned timber. Consideration should be given to the following:

members with width to thickness ratios less than approximately 3:1 are less prone to cupping, e.g. exposed decking.
narrower board products expand and contract less than wider boards. Flooring of narrow boards will have more gaps, but they will be smaller. Less stress is induced in fixings where shrinkage occurs in narrower boards.

Members with small width to thickness ratios are usually less prone to the effects of bowing.

Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.