No matter what sort of roofing material is used, do not forget the great
importance of the roofing foundation (Figs. 296 and 298). If the
foundation is poor or uneven the roofing will be poor and uneven, even if
only the best roofing material is used. The sheathing boards should be
matched if possible and of uniform thickness, laid close, and free from
nails, protruding knots, and sharp edges. Do not use green lumber; the sun
is almost certain to shrink and warp it. Sometimes it will even break the
roofing material. On very particular work, where the rafters are wide
apart, the best builders recommend laying a course of boards over the
planking at right angles to it.