In the preparation of the ordinary slate roof, the rafters should be covered with ⅞-inch thick, tongued-and-grooved roofing-boards. In order to prevent buckling, if they should swell with dampness, it is essential not to drive the joints between boards up too tight. As these boards are surfaced only on one side, this side is laid against the rafters and the tongues are placed upward so that a better shedding of water is secured. Good nailing with tenpenny nails is important, and all joints at ends of boards should be made over rafters. A cheaper but not so good a bed for the slate can be made with common, unsurfaced sheathing-boards. In the cheapest kind of work sheathing-boards are not used, but only shingles lath. Over the top of this rough boarding should be tacked 11 pounds per 100 square feet slater’s roofing felt, laid horizontally and lapping joints 3 inches. The usual commercial sizes of slates are ³/₁₆ inch thick, and of the following standard sizes: 6 by 12 inches, 7 by 12 inches, 8 by 12 inches, 7 by 14 inches, 8 by 14 inches, 10 by 14 inches, 8 by 16 inches, 9 by 16 inches, 10 by 16 inches, 12 by 16 inches, 9 by 18 inches, 10 by 18 inches, 12 by 18 inches, 10 by 20 inches, 12 by 20 inches, 11 by 22 inches, 12 by 22 inches, and 12 by 24 inches. They have two holes in each piece for nails, which nails should be 1-inch copper slater’s nails, or 3d galvanized slater’s nails for cheaper work. The first course should be started 2 inches below the line of the sheathing-boards at the eaves, and the necessary tilt is given with a ³/₁₆ by 1 inch cant strip. A double thickness of slate is used for the first course, the upper layer breaking joints with the lower. At the gable ends the slate should not overhang more than 1½ inches. The exposure to the weather for courses of slate is determined by taking one-half of the length of the slate minus 3 inches. The ridges of the roof may be finished in two ways, either with the combed ridge or the saddle ridge. The combed ridge is formed by projecting a finishing course and a combing course of slate on the north or east side of the roof 1½ inches beyond the top and combing course on the opposite side of the roof. Both courses are laid with slate set lengthwise, the length being twice the width of the slate used on the roof. This last course is laid in elastic roofing cement, and the nails are also covered with it. The saddle ridge is formed by alternately butting the ends of the top course on one side with the top course on the other, and then doing the same with the combing course. This makes a zigzag joint which is closed by the elastic cement used in setting.