Improperly dried or stored wood
may suffer from cracks and deformations due to shrinkage, while it
may twist, bow, crook or warp caused by a combination of shrinkage
and swelling. This is caused by the moisture content (MC) of the
wood. The higher this content the greater the chance that one or
more of these negative effects will occur. In addition wood taken
from the outer rings of the log (called sapwood) is more sensitive
to distortion than wood taken form the core of the log (heart wood).
Most countries require that at least the
boards shall be kiln dried to a moisture percentage of maximum 15%.
in such case our company needs to issue a certificate proving that
kiln drying has been applied indeed. Without this certificate you
may be unable to import the wood. We strongly advise you to contact
the local authorities to verify this requirement (normally the
ministry of Forestry or Agriculture)
EFFECTS OF THE
PROBLEM RESULTING FROM IMPROPER DRYING OR STORAGE.
Distortion of flat
and square wood as affected by the annual rings. The white boxes
indicate from which part of the log the wood was cut. The
figures A thru C show the shrinkage in drying from green to
A = boards taken
from the outer rings of the log (sapwood)
B = square beam
taken from the outer rings of the log (sapwood)
C = board taken
from the core of the log (heart wood)
D = square
beam taken from core of the log (heart wood)
The moisture content is measured as the ratio of the
weight of the free water in a given piece of wood to the weight of the wood
when it is completely dry (oven dried, or kiln dried) and is usually
expressed as a percentage.
The "green" wood of a freshly felled tree may have an MC
anywhere in the range 30% to over 200%, depending on the species (hardwood
(1) is far less receptive for a high MC than soft wood). Almost all of his
water must be removed from the wood before it is fit to be used for its
particular intended purpose (for instance; wood for furniture shall be dryer than wood for
(1) Teak, Merbau, Iron wood, Bangkirai, Nyatoh, Kempas,
Camphor and Keruwing are
the hardwoods that we use. Coconut wood is a soft wood.
Wood for structural purposes is considered dry when a moisture content lower
than 20% is achieved. This implies that all of the free water has been
removed and only bound water remains, starting some 10% below the fiber
saturation point. (Saturation point is defined as the boundary between free
water and bound water, which is around 30
FREE WATER AND
BOUND WATER IN WOOD
Cracks and warping will occur in the saturation zone (between 30% and
100%, shown in red), NOT in the bound water zone
provided the wood is properly dried in
an autoclave (kiln dried), a process that may take 4 to 7 weeks,
depending on the type of wood. Air drying, however, is largely preferred
over kiln drying. The set back is that such will take significantly longer,
a process which we often use during the progress of prefabrication when a
until takes months to built.
Furthermore, wood is a cellulose material which behaves
somewhat like a sponge, so that even wood which has been kiln dried down to
say 7% may in fact later reabsorb water from the atmosphere so as to reach
an equilibrium state. Actually all
wood is constantly gaining or losing water to or from the environment, in
other words, the moisture content of wood changes as the relative humidity
changes. Coats of varnish or paint can slow the process but cannot stop it.
As the moisture content of improperly dried wood (or
green wood) changes, so does the wood expand or contract, potentially
producing all manner of disastrous defects (wood warping and wood cracking
Most of the wood for our bungalows and cottages is kiln
dried to an MC of 12%. When arriving at destination this MC may have
increased to a maximum of 20% depending on the humidity of the environment.
This MC percentage of 20% is still 10% lower than saturation point, thus in
the bound water (technically dry) zone and in conclusion
safe against cracking and warping.
NOTE: When building in an
seasonal environment interspersed with low humidity and low temperatures in
winter and higher humidity and high temperatures in summer, all boards
must have an MC of 10%, whereas beams and columns shall be dried up to 15 %
to 20%. When this is not achieved boards will crack and warp, while beams
and columns will crack and/or deform.
The nightmare of the wood supplier............our digital moisture
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