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Sawn
timber products
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Sawn solid timber products are used for engineering,
construction, flooring, lining, joinery, furniture and crafts.
For a given product, many factors will affect a log’s value
at the mill. These include:
• species
• diameter
• age
• straightness
• length
• presence of product or processing related defects
• growth stresses.
All of these can influence processing costs and the recovery
of particular products. A log that best meets market specifications
will be worth much more than a poor log at the mill door and
this will probably be reflected by market interest and price.
Before looking at the factors that may affect sawlog quality
it is worth considering the definitions of particular terms
such as:
Recovery
Backsawn
and quartersawn
Natural
growth stresses
Juvenile
wood
Knots or
branch stubs
Other defects
and effects
Log shape and tree form
For sawn timber production it is often critical
that the logs come from straight, upright trees. Leaning trees
are susceptible to compression in softwoods and tension in hardwoods.
This can cause the quality to be downgraded and lead to high
losses of useable timber during sawing and drying. Leaning trees
also tend to develop a sweep that makes it difficult to produce
long, straight logs. Sharp bends and kinks are also a problem.
Although small bends in young trees may grow out over time,
a wandering pith will lower recovery of the better sawn timber
grades. The log’s pith or heart is commonly boxed out and
discarded due to problems with drying boards containing the
pith. The degree of taper can also be critical. Logs that are
near perfectly cylindrical are clearly best. Where the diameter
decreases rapidly up the stem, the recovery of sawn timber will
be greatly reduced. Because some log buyers determine volume
on the basis of the small end diameter excessively tapered logs
are severely downgraded.
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Log length
Depending on the sawing equipment and product
options, a sawmill should be able to specify a minimum and maximum
log length. Most prefer longer logs because they are cheaper
to mill and dry. In some cases sawmillers may cut longer logs
in two if there is concern about growth stresses or excessive
taper.
Log diameter
Once a tree is felled and cut into lengths
it is the diameter of the log, not the height of the tree, which
will be most important to the miller. Generally, large diameter
logs give greater sawn timber recovery, at lower fixed milling
costs with a greater proportion of timber achieving the higher
value grades. Larger logs are also easier to quartersaw and
may be less prone to growth stress problems. Larger dimension
boards can only be cut from large diameter logs.
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