Article by Roger Galpin, Product Certification Manager, BM TRADA, published in TRADA Timber Industry Yearbook 2002.
What is Dry Graded Structural Timber?
In order to answer this question, we need to understand one of the most significant, yet least understood characteristics of timber - moisture content.
When a tree is first felled and converted, the timber contains a large proportion of water. The amount of water can be expressed as a percentage of the dry weight of the timber. This is referred to as the moisture content of the timber, and hence it is possible to have a moisture content that is greater than 100%.
As timber dries, it reaches a point at which all the free water is lost and only the water in the wood cell walls remains. This is referred to as the fibre saturation point, normally around 25-30%. Below fibre saturation point, shrinkage and distortion can occur as the bound water in the wood cells of the timber is given up.
Ideally, timber should be graded and installed in a building at a moisture content that is appropriate for the ambient conditions, that is, at a point where there is no further gain or loss of moisture; an equilibrium moisture content. As shown in Table 1, this level will vary depending on the service conditions.
Table 1 Typical equilibrium moisture content of components in service
Service environment | Moisture content % |
|---|
External joinery | 15 - 18 |
Roof timbers | 14 |
Intermediate floor | 12 |
Of course, this ideal situation is seldom possible, or viable in practice. However, in order to minimise movement (shrinkage) in service, dry graded structural timber must be graded and supplied at a maximum moisture content of 20%.
Why Specify Dry Graded Structural Timber?
In order to comply with the building regulations, suppliers of dry graded structural timber must ensure that the timber is dry (maximum 20%), both at the time of grading and when supplied for use in construction.
Compliance with the mandatory requirements of the building regulations is achieved by meeting the recommendations contained in BS 5268-2: “Structural Use of Timber” and the Standards that it refers to with respect to the grading of softwood (BS 4978 and BS EN 518 – visual grading of softwoods, and BS EN 519 – machine grading).
Dry graded structural timber affords improved performance in terms of dimensional stability, fewer fissures, easier machining, and removes the risk of fungal decay and corrosion of fixings.
