Timber Defects
Timber is subject to many defects, which should, as far as possible, be cut out during
its conversion. These defects can be divided into two groups: timber seasoning defects
and natural defects.
What does ‘seasoning’ mean? Seasoning refers to the controlled drying by natural or
artificial means (see below) of converted timber sections. There are many reasons why
seasoning is necessary, these are the main ones:
To ensure the moisture content of the timber is below the dry rot safety line of 20 per
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cent.
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To ensure that any shrinkage takes places before the wood is used.
Dry wood is easier to work with than wet wood.
Using seasoned wood, the finished article will be more reliable and less likely to split
or distort.
In general, dry wood is stronger and stiffer than wet wood.
Wet wood will not readily accept glue, paint or polish.
Seasoning defects
Distortions – These may be the result of an inherent weakness in the timber or bad
conversion, but often they develop during seasoning through poor stacking or uneven air
circulation.
Bowing is a curvature along the face of a board, and often occurs where insufficient piling
sticks are used during seasoning.
Springing is a curvature along the edge of the board where the face remains flat. It is often
caused through bad conversion or curved grain.
Winding is a twisting of the board and often occurs in wood that is not converted parallel to
the pith of the tree.
Cupping is a curvature across the width of the board and is due to the fact that wood
shrinks more tangentially than it does radially.
Shaking or splitting. These are fissures (separation of wood fibre) that develop along the
grain of a piece of wood, particularly at its ends, and are the result of the surface or ends of
the wood drying out too fast during seasoning . The term ‘fissure’ also includes resin pockets
when they appear on the face or edge of a board.
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Collapse. This is also known as wash-boarding and is caused by the cells dehydrating and
collapsing through being kiln dried too rapidly in the early stages.
Case hardening. This is the result of too rapid kiln drying. In this case the outside of the
board is dry but moisture is trapped in the centre cells of the wood.
Stick staining is the result of using a different species of wood for the piling sticks to that
being seasoned. It can sometimes be removed when boards are processed, but total success
depends on the depth of penetration.
Natural defects
Shakes are caused by a separation of the wood fibres along the grain developed in the
standing tree, on felling or prior to seasoning. The shake is formed as a result of stress relief,
causing a longitudinal crack radiating from the heart and spreading through the diameter of
the trunk.
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Heart shakes are cracks along the heart of a tree and are probably due to over
maturity.
Star shakes are a number of heart shakes that form an approximate star.
Radial shakes are cracks along the outside of the log and are caused by the too
rapid natural drying of the outside of the log before it is converted or by heavy felling onto
hard ground.
Cup shakes or ring shakes are a separation between the annual rings and are
normally the result of lack of nutriment. It is also said to be caused by the rising sap freezing
during early spring cold spells.
Waney edge is where the bark is left on the edge of converted wood.
Knots are the end sections of branches where they grow out of the trunk. Knots can be
considered as being either live knots (i.e. knots that are firm in their socket and show no
signs of decay, also known as sound or tight knots) or dead knots (i.e. knots that are
separated from the surrounding signs of decay).
Upsets. This is a fracture of the wood fibres across the grain also known as thunder shake.
Lightning damage sometime during its growth can cause upsets but the severe jarring the
tree receives when being felled is the main cause. This is a serious defect, most common in
mahogany and is not apparent until the wood has been planed.
Sloping grain is where the grain does not run parallel to the edge of the board and is often
caused by bad conversion or by bent logs. When the sloping grain is pronounced, the defect
is called short graining. This seriously affects the strength of the wood and it should not be
used for structural work.
Reaction wood is produced in timber that has had to grow in a leaning position either on a
slope or against strong prevailing winds. The tree will attempt to produce extra growth on
the trunk to counteract the force of gravity or wind force.