A number of possible shortcomings in the wood affect its suitability as an exterior cladding material.
Knots, slope of grain and the differences in expansion and contraction in the three directions can lead to various distortions (Figure 8.6):
• twist (warping): the wood is distorted about the axial direction;
• cup (concave or convex): the sawn section acquires a dish-shaped cross-section. This phenomenon is sometimes known as'panning';
• spring: deviation of the straight line measured over the smallest axis;
• bow: deviation of the straight line measured over the largest axis.
Elements with a round or rectangular cross-section tend to distort upon drying out, depending on the wood's original position in the tree trunk. Circular cross-sections then become elliptical, square cross-sections become rectangular or even rhomboid (Figure 8.7).The pith of the tree is particularly susceptible to splitting and its use should therefore be avoided.Thin, wide sections will curve inwards towards the heart of the tree.This must be borne in mind as floor; roof and cladding elements are best applied so that any future curvature is always convex to the viewer This not only improves the appearance but ensures that the joints are more secure. Sections are sawn from the trunk slightly oversized (2-3 mm) on each side (nominal size), and are then planed to the exact (finished) size before actual incorporation into the structure. If the timber is then sawn again, further distortion may occur, especially in hardwoods like iroko. Grooves routed into the reverse of the element may reduce the risk of curvature.
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