Moisture and Seasoning of Timber

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Moisture content is also a vital factor in designing and specifying timber for structural use as it has a direct effect on strength, dimensions and durability.

 

The typical moisture content of freshly felled timber is 100% but varies depending on what time of year the tree is felled. When timber dries dimensional change (shrinkage) occurs.

 

The moisture content of timber can affect the strength and stiffness properties and, generally, timber strength increases with decreasing moisture content values.

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Wood just felled in ‘green’ condition must be dried out, or seasoned,  in order to be used as construction timber.  Typically wood is dried either in the open air (which takes anything from a few months to a year) or, if speed is important, in vast heated ovens called kilns which cuts drying time to days or weeks.

 

The relative humidity of the kiln is controlled in order to reach the wood’s equilibrium moisture content (EMC); a balance between the moisture content (MC) of the wood and the relative humidity (RH) of its surrounding environment, depending upon the end wood product’s application specification.

 

Once EMC is achieved a wood’s MC can vary with slight changes in RH without jeopardising the strength and structural integrity of the end wood product.

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Most structural timber in construction is softwood, either redwood or whitewood. Building regulations require that all structural timber used in buildings has to be strength graded at a moisture content appropriate to the service environment in which it will be used, and marked ‘dry’ or ‘kd’ (for kiln dried).

 

The timber must also be stamped with the appropriate grade marks i.e. C16 or C24. For internal construction the building regulations require that structural timber must satisfy the dry grading requirements which are that the timber is dried to an average moisture content of 20% or less, with no piece exceeding  24% at the time of grading.

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There are a number of advantages of dry graded structural timber :- reduced future shrinkage, lighter, stronger, less chance of fissures checks and splits, less chance of fungal attack, less chance of bow twist and spring, easier to machine, better finish when machined, easier to glue, easier to paint, easier to treat with a preservative, easier to stain, and it does not corrode fixings.