Grading: Strength and Appearance
Strength grading allows efficient use to be made of structural timber. Both visual grading and machine grading are used and both are equally valid. Visual grading assesses the size, frequency and positions of defects in the timber, such as knots, wane and sloping grain, and compares them to grading rules which define the allowable size, type and number of defects for each grade. Visual grading of softwoods is carried out to BS 4978, or for softwoods imported from Canada or the USA to their own standards; NLGA and NGRDL respectively. Machine grading measures the resistance of the timber member to flexing, which gives a measure of the strength of the piece. Machine grading is carried out to the requirements of BS EN 519.
All strength grading, whether visual or machine, must be carried out under the control of a third party quality assurance scheme (eg the BM TRADA Certification Ltd Q-Mark). All structural timber members must be clearly marked with their strength grade and/or class and a quality assurance mark.
The strength of timber is related to the species, so structural graded timber is grouped into strength classes which contain grades and species with similar strength properties. The strength classes are based on the characteristic values for bending strength - the class numbers relating to the bending strength in N/mm2 - and are defined in BS EN 338. Visual grades of individual species or species groups of timber are assigned to a strength class on the basis of the species’ strength and the grade of the piece. Machine grading allows each piece of timber to be assigned to and marked with its appropriate strength class.
Grading based on appearance is carried out by most producer countries to their own rules. These commercial grades are used in international and local trading. They are not appropriate for use in specifying timber for buildings.
Timber for use in joinery is selected and graded into classes based on the size and location of knots and other defects. The classes are defined in BS EN 942.
