Non-poisonous timber protection
Practical Action
Reduction of moisture content
Timber should be cut when its moisture content is the lowest (towards the end of the dry
season). Allowing the felled tree to lie for some time with all its branches and leaves
accelerates the drying process.
Seasoning timber is the process of drying sawn timber under a roof for protection from
the rain. The saturated air
around the timber is
removed by fresh air, which
is circulated naturally, or
forced through the gaps
between the pieces of
timber. Seasoning should be
done to a moisture content
in equilibrium with the
atmosphere in which the
timber will be used, in order
to avoid excessive moisture
movement after
construction. Kiln seasoning
is faster than air seasoning,
and provides timbers of
uniform moisture contents
by controlling the air
circulation, humidity, and
temperature within the kiln.
Recently, solar seasoning
kilns have been developed
(Figure 2). The interruption
of the drying process after
sunset enables the moisture
from the centre of the wood
Figure 2: A solar Kiln
to come to the surface, thus
preventing excessive tension between the surface and the interior, which can cause
distortions and splitting.
Avoidance of distortion and splitting
Distortion and splitting are caused by wood shrinking. This varies according to the
direction of the shrinkage. Longitudinal shrinkage is negligible - about 0.1 to 0.2 per
cent. Radial shrinkage is about 8 per cent from the 'green' to the dry state. The
corresponding
tangential shrinkage
is about 14 to 16 per
cent (Figure 3).
Tangential
shrinkage
The stacking of sawn
(14 – 16%)
timber for seasoning
and for storage
afterwards requires
careful horizontal
Radial
Shrinkage
(8%)
placement on equally
spaced battens to
avoid sagging and
differential stresses.
Figure 3: Timber shrinkage
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