On roofs which pitch more than five inches to the foot, slates may be used. In this case the boarding should be matched and covered with tarred paper. On this surface the slates are laid in layers beginning at the eaves with a double course, and working upwards to the ridge. The gauging of the courses is determined by the "head cover," that is, the distance which the top of the slate is overlapped by the second slate above. (Fig. 155.) This lap should not be less than three inches.

The usual sizes of slates are eight by sixteen and ten by twenty inches, and they run about 3/16 to 1/4-inch in thickness. Slates should be put on with tinned, galvanized, or copper nails, two to each slate, and care must be taken that the nails are not driven in hard enough to crack the slates, and yet hard enough to prevent them from rattling. The nails should have large heads and be 3-penny or 4-penny nails. Where slates are cut, as against hips or valleys, care should be taken that each slate receives two nails. For thick slates the holes should be drilled and countersunk. For first-class work the slates, for a distance of two feet each side of valleys and above gutters, should be "rendered," i.e., bedded in elastic cement, and the same precaution taken for a foot in width against hips, ridges and all vertical parts. If slating is required for very flat slopes, the whole should be laid in cement.

Fig. 155. Head Cover of Slates.