Nearly all fireproof office buildings, apartment houses, hotels and warehouses have "flat" roofs, pitched just enough - generally from ¼ to ½ an inch to the foot - to cause the water to run to the lowest point. It is easier to make a flat roof thoroughly fireproof than it is a pitch roof, and the flat roof is also much less expensive.
The usual, and also the best, method of constructing flat roofs on fireproof buildings is to build the roof in the same way as the floors, giving the beams the same pitch as the roof. If the filling between the beams is of hollow tile, segmental arches, or flat arches with raised skewbacks, may be used with economy.
When any of the patented systems of fireproof construction is used, the roof, if flat, is almost invariably constructed in the same way as the floor, only using a little lighter section.
The roofing may be either of tin, copper, rock asphalt or composition, finished on top with gravel or vitrified tile set in Portland cement. Coal tar, pitch and asphalt have a natural affinity for cement or terra cotta, and adhere readily to them without the use of fastenings. If a tin or copper covering is to be used, porous tiling is especially adapted for the beam filling, as the nails for the tin cleats may be driven directly into the tiling. Before applying the tin the entire surface of the roof should be plastered smooth with ¾ of an inch of cement mortar to form a smooth, hard surface on which to hammer down the tin. Thin, hollow tiles, set between 3 X 3-inch T-irons, are also occasionally used for roofs.
Whatever kind of tiling or filling is used it should be of such construction that the bottom flanges of the beams or T-irons will be well protected, and if tiling is used it should receive a heavy coat of plaster under the beams, if not elsewhere. The supporting girders and columns should also be well protected, either with hollow tiles, concrete or plastering on metal lathing.
For mansard roofs the most economical method of construction is by using I-beams, set 5 to 7 feet apart, and filled in between with 3-inch hollow partition tile, provision for nailing slate being made by attaching 1¼ x 2-inch wood strips to the outer face of the tile, the strips being set at the proper distances apart to receive the slate, the spaces between the strips being then plastered flush and smooth with cement mortar. In case of a severe conflagration the slate would probably be destroyed, and the wooden strips might be consumed, but the damage could go no farther. In place of partition tile porous terra cotta bricks or blocks may be used for filling between the I-beams. For roofs where the pitch is not over 450, 3 X 3-inch T-irons, set 16 inches between centres and filled in with slabs of porous terra cotta, make a very desirable roof. If slates or roofing tiles are used they may be nailed directly into the porous tiles, or, if it is desired to use hollow tile, strips of wood may be nailed to the tile for receiving the slate and the spaces between the strips filled in with cement.
All truss members, purlines, etc., should be protected from fire and heat either by wire lathing or by porous tiling, covered with a heavy coat of plaster. Probably the best and most thorough method of protecting truss members is by first covering them with 1½-inch slabs of porous tiling and wrapping them securely with stiffened wire lathing, which should then be covered with a heavy coat of cement plaster.