Question

What role, if any, does Moisture content (water) play in timber which is used for Structural purposes?

Answer 1

MOISTURE CONTENT-:
In the timber industry the amount of moisture present in timber (or its moisture content) is defined as the mass of water present in the timber divided by the mass of the timber with all water removed, expressed as a percentage. . Hence the following equation is used to determine the moisture content of timber:-
= “initial mass” - “oven dry mass” x 100%/ “oven dry mass”

The structure of the cells in timber can be likened to a number ofdrinking straws glued together. If the straws were full of water it could be expected that the mass of water contained in the straws would be greater than the mass of the drinking straws alone. In such a case the moisture content as calculated above would exceed 100%. In a tree the moisture content may be as low as 40% but can be ashigh as 180%. Green off saw timber could therefore have moisture contents of 180%, which means the timber contains 1.8 kg of water
for every 1.0 kg of dry timber that is present. In softwoods such as radiata pine and Araucaria average moisture contents of 180 % or more often occur. In many of our common hardwoods the moisture content may be no greater than 70%. Cypress, a softwood that grows in drier areas, may only have average green moisture contents
of 45%.

THE DRYING OF TIMBER-:
Seasoning is the process by which moisture is removed from timber (i.e. drying) and green timber may be either air dried, kiln dried or a combination of both. Timber that is stacked to allow air movement between each layer of timber can be air dried by leaving it out in the open for some months or it can be dried more quickly in the controlled conditions of a kiln. When we refer to seasoned timber, we are usually referring to timber that has moisture contents in the range from 9% to 14%. This range has been chosen because timber  will usually remain within this moisture content range, when used internally.

THE EFFECTS ON TIMBER PROPERTIES RESULTING FROM DRYING-:
Although some products are manufactured from green timber, it is necessary to dry timber for many applications. When timber is dried, a number of benefits are achieved as follows:-

• greater dimensional stability with the timber less prone todistortion and smaller shrinkage gaps in applications such as flooring.
• less susceptible to insect attack and prevention from fungal attack providing the timber remains dry
• improved strength and stiffness properties enabling the timber to take higher loads with less deflection.
• reduced weight making it lighter to handle
• better machining characteristics providing a smoother machined surface
• acceptance of a wider range of glues and finishes
• ability to accept preservative and other treatments
• It is for these reasons that much of the timber used for both internal and external applications has been dried.