BS 4978:1973

Specifies structural timber assessed by visual or machine stress grading. Requirements are given for sizes, deviations in size, processing reductions, knots, fissures, slope of grain, wane, rate of growth, resin pockets, distortion, wormholes and other defects. Illustrations show the principles of knot projection to estimate the knot area ratio in visual grading and the measurement of other characteristics affecting strength.