Seasoning of timber refers to the processes used in reducing the moisture content of the timber as near as practicable to the mean value of the moisture content it is expected to attain in service. Thus, the reason for seasoning of timber is due to the effect of moisture on timber and these effects include:
Below fibre saturation point (moisture content of about 27 – 30%), all strength properties of timber except toughness increase with a decrease in moisture content. There is no noticeable effect for moisture changes above fibre saturation point.
Moisture Exchange with the Atmosphere
Wood is hygroscopic and tends to attain a moisture content that is in equilibrium with the surrounding atmosphere. The moisture content of wood is affected by atmospheric temperature and humidity; and indoors, by artificial cooling, heating, and ventilation.
At any moisture content below 27 – 30%, moisture changes cause shrinking and swelling in wood. Shrinkage is the more serious problem. The amount of shrinkage depends upon the moisture loss, the timber species, the direction of the growth ring, etc. Changes in dimensions are frequently accompanied by changes in shape, i.e. distortion of members.
In larger sizes, particularly in lower grade material, drying, and consequent shrinkage are often accompanied by the formation of checks and end splits, the latter defect being particularly important in influencing joint strength.
Wet timber is very prone to attack by fungi, but it offers much greater resistance as the moisture content is reduced to below 25oC. Such timber is not immune to attack by insects such as termites and lyctus against which other protective measures such as impregnation with suitable wood preservatives should be taken.
After Seasoning, What Next?
Generally, all seasoned timber should be preserved according to the following conditions;
a. The magnitude of the hazard of insect and/or fungal attack in the locality.
b. The position of the timber in construction, e.g.
i. interior timber
ii. exterior timber not in ground contact but exposed to the weather
iii. exterior timber in ground contact or in the ground
iv. exterior timber in fresh water, and exterior timber in seawater
v. timber in cooling towers
c. The species of wood used and whether or not sapwood is present.
d. The proposed lifecycle of the timber.
e. The cost of repairing and replacing defective parts.