There has been much made of the so-called European method of laying slate roofs in recent years, but this type of roof costs more than the ordinary slate roof, since special heavy slate is used at the eaves, and the weathering is reduced as the courses approach the ridge, and special care is taken in blending colored slates. While this type of roof is very beautiful, it is really, from a point of view of cost, rather out of the race when applied to the small house, for it will be hard enough to stretch the estimates of the small house to include even the ordinary slate roof.
In the preparation of the ordinary slate roof, the rafters should be covered with 7/8-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, un-surfaced 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 1 1/2 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 1 1/2 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.
The Boston hip is the best. Each course is brought at its upper or nailing edge to within 2 inches of the hip line. A small strip of slate then finishes this off by fitting to a mitre cut made on a slate set parallel with the line of the hip. These hip slates have the lower corner of their butt ends on a line with the next lower course, and they are lapped with the opposite hip slate and made tight with roofing cement.
Hips may also be finished by bringing each course up to the hip line, and mitring them with the opposite courses on the other side of the hip.
Valleys should be lined with 16 ounces copper, 4 pounds lead, IX tin, or a prepared roofing roll weighing 37 pounds per 108 square feet. Measuring from the centre of the valley to the edge of the slate along the valley, this distance should be 2 inches at the top and increase 1/2 inch in every 8 feet length of valley, to widen it out toward the bottom. The flashing should extend up under the slate on either side about two-thirds the width of the slate used. If 8-inch by 16-inch slates are used, this means that the distance should be about 5 inches. If the slopes of the two intersecting roofs are different, and there is a chance that the volume of water sweeping down the larger and steeper incline may be forced up under the slate at the valleys, the metal lining should be crimped up (inverted V-shape) at the centre, 1 inch, to form a little dam against the rush of the flood.
Flashing used against chimneys, dormers, or other vertical walls should be bent up 4 inches and extend into the slate courses 4 inches. All vertical flashings against masonry should be cap-flashed and made tight with elastic cement. The cap-flashing should extend down over the flashing 3 inches, and be inserted into the masonry at least 2 inches.
Sometimes the closed valley is designed for slate roofs, in which case the valleys must be rounded out with the roofing-boards, blocked to position. The slate courses should be carried around this curved valley, but each course in the valley should be covered with flashing just under the lap of the course above and extend up toward the nails.