In a previous paper in this series it was concluded that the warp (twist, crook and bow) of structural timber products is of utmost importance for end-user satisfaction. In this paper, a study of the spatial variation of warp of fast-grown Norway spruce is presented. The material, 372 studs (45 x 70 β’ 2900 mm2), was cut from different radial and longitudinal positions in trees from two stands.
The results indicated that twist and crook were at maximum near the pith. However, twist decreased much more rapidly with the distance from pith compared with crook. Crook associated with core studs was larger in the butt log than in the top log, while twist and bow in core studs appeared not to vary longitudinally. Heartwood formation in the butt log juvenile core did not diminish warp propensity. Twist was rather well correlated to the distance between pith and centroid of cross section. The ratio of grain angle to pith distance was the best parameter to predict twist. Presence of compression wood increased bow and crook significantly, while ring width, density and knot area ratio did not substantially contribute to explain warp variation.
A moisture content change from 18% to 12% decreased the overall acceptability, expressed by stud grade yield, from 76% to 43%. Thus, timber products should be delivered at a moisture level close to the one expected in the finalized building in order to avoid excessive warp after delivery.