the wood and wood products industry
Timber Moisture
There is an intrinsic relationship between moisture content and the performance of timber and wood based products. This is because wood is a naturally hydroscopic material and will gain or lose moisture in sympathy with its surrounds. Timber shrinks as it dries and may swell again if wetted. However, if dried to a moisture content close to that which it will reach in service it will not swell or shrink significantly. Strength increases as timber dries and dry wood not susceptible to any form of fungi attack.
The correct specifications and, more importantly, the correct moisture content at installation, are absolutely essential to ensure the product you have chosen performs to the highest standard. For instance in "covered and generally heated end-uses" BS 5268 part 2 recommends structural timber is at a moisture content of 20% when installed. By comparison, recently felled trees and 'green' timber can have moisture contents as high as 200% of the weight of the dry wood. Don't take the risk, specify the correct moisture content and make sure the goods delivered to site meet that requirement.
Unfortunately, poor site practice can easily negate the hard work put in by designers and suppliers. Timber and wood based products need to be given adequate protection from the elements between delivery and installation. Where appropriate wood based products may also need to be conditioned before use. "Conditioned" is the process of allowing timber or wood based products to stand in their end use environment for a period of time prior to installation. The procedures and equipment required to check timber is at the correct moisture content are simple and inexpensive.
Kiln Drying
Kiln drying allows faster drying under controlled conditions to minimise potential degrade of the timber.
The most common method is to heat the wet timber in an enclosed kiln to a temperature of 60°C-70°C and extract the moisture from the warm air. The heating regime may last several days and is determined by the species of timber and target moisture content. Timber can be kiln dried to moisture contents lower than can be attained with air drying.
Softwood for structural uses which has been designed on a dry basis must be strength graded at an average moisture content of 20%.and marked DRY or KD (kiln dried) within the structural grade stamp.
For more information on moisture and kiln drying please click here.
