46 MOLDING CONCRETE ROOFS ployed with success upon several structures where a fire-proof and all-concrete roof was desired. While there are various methods of providing the joints to the slabs, the one shown in Fig. 13 is the most simple and successful. The slabs are molded in forms that produce a channel at each side, so that each slab laps over the edge of the one above and below same; this lap should not be less than 11", and is better when made 2" in width. The joints are further protected against moisture by cementing the slabs together when the roof is laid. The rafters are constructed in the manner illustrated, with a channel or recess into which the slabs rest. This may be in the form shown, or as a groove or channel into which the edge of the slabs are placed, thus wedging them securely in position. The rafters must be amply reinforced as well as the slabs, as a roof of this type is very heavy and would not be practical except upon such structures as have walls of ample strength to carry the weight. CONCRETE SLATE OR TILE ROOFS Roofs of concrete slate have been in use for several years, and when these are properly constructed give universal satisfaction. A concrete slate roof or one of concrete roofing tile is less in weight than a clay-tile or slate roof. Each 100 square feet of claytile will weigh from eight hundred to one thousand and six hundred pounds; slate, from seven hundred and fifty to nine hundred and fifty pounds, while 48 MOLDING CONCRETE ROOFS the weight of 100 square feet of concrete roofing tile will vary from six hundred to six hundred and fifty pounds, and that of concrete slate " to 8" thick will vary from three hundred and fifty to four hundred pounds. This difference in weight is of consideration, as it not only lowers the cost of construction but insures greater durability to the roof, as the walls are not required to carry as heavy a load. The material cost of concrete slate is also of great interest. This varies from $2.15 to $3.00 per square when made " thick. Each slate is reinforced with a square of metal-lath 12" x 12", thus insuring it against breakage. The difference in cost is caused by the price of cement and other materials in various sections of the country. To enable an accurate estimate of the material cost, it may be safely estimated that one-half barrel of Portland cement, 8 cubic feet of sand, 11 square yards of metal-lath, and 6 pounds of mortar color will produce 100 concrete slate 14" x 14" square and " thick. These, when laid on the roof with a lap of 2", will cover one square, or 100 square feet. While there is a difference of opinion as to the best proportioned mixture for concrete slate and roof tiles, the one most universally employed for slate of 1" to " in thickness is a mix of 1: 2, using sharp well-graded sand running from coarse to fine particles. This must be used as wet as possible, so that the mortar will adhere strongly to the reinforcement. There is a wide difference between molding flat sheets and concrete block work, as the mix may be made far more moist and yet work with success. A mixture of 1: 3 has also been employed for tile, where there was more body to give strength, than in the thin sheets. Proportions leaner than 1: 3 are not to be recommended for this work, as the slate or tile must be molded as thin as is practical to reduce the weight of the roof; hence the sheets must be made as dense and strong as possible. MACHINE OR MOLD FOR CONCRETE SLATE The machine illustrated at Fig. 14 permits the rapid molding of concrete slate in any thickness from 1" to ", as may be desired. While the construction is designed to be of lumber, treated with a water-proofing paint, yet the same plan requires but slight changes to be constructed of metal. As shown in Fig. 14, the mold is made of two plates that join together in the center and are the exact outline of three complete slate; these plates are also the exact thickness of the slate to be molded. The pallet is placed directly under these two plates, and so forms the bottom of mold. As illustrated in Fig. 14, the mold is ready for the concrete to be placed with the 12" x 12" squares of metal lath in each one; when tamped and finished, the two plates are drawn to each side, thus releasing the pallet, upon which the slate remain until cured. This requires a pallet for each three slate molded; but the cost and labor of preparing these is repaid in the |