This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Flat Roofs. The roof of a fireproof building is usually flat, with a pitch of from 1/4 inch to 1/2 inch to the foot. The advantage of a flat roof is the ease of making it fireproof, and also the smaller cost as compared with pitch roofs. The roof is built in the same manner as tile floors, the tiling being carried on light T or I beams, and the supporting columns having a lighter section than those under the floors.
When the tile or other material forming the bearing surface is in place, it should be plastered over with cement mortar, to give it an even face, ready for the exterior covering. The latter may be either of tile, set in cement, or of tar and gravel, asphalt, copper, iron, or tin. The use of porous tile is recommended when copper or tin coverings are to be used, as nails may be readily driven into it. A very essential point is that the bottom flanges of all I or T beams be completely enclosed by fireproof materials; when tiling is used, a heavy coat of plastering should be placed on it, especially under the beams. All supporting girders and columns should also be well protected, either by hollow tiling, or by plastering on metal lathing.
Mansard And Pitch Roofs. For mansard and other roofs having a very steep pitch, the usual construction consists of I beams set from 5 to 7 feet apart, the intervening spaces being filled with 3-inch hollow partition tile. If slate is to be used in covering, nailing strips are fastened to the tile, and the surface is brought up flush with cement mortar, upon which the slate is laid. The strips are unnecessary, of course, if porous tiling is used. For roofs having less than 45° pitch, plates of porous tile, supported on 3"x3"T beams, are very satisfactory. All parts of the ironwork should be well protected from fire by casing them in porous tiling or wire lathing, covered with a thick coating of plaster.
192. Ceilings under flat roofs are usually similar in construction to those beneath the floors, but all parts are made quite light. Under pitched roofs, however, when a flat surface is necessary, the ceiling is usually suspended by rods attached to the roof framing. These carry angles or T bars, to which is fastened the wire netting holding the plaster. Such ceilings weigh about 12 pounds per square foot, exclusive of iron.
Sometimes light tiling is used instead of the metal lathing, in which case the porous variety is the best. In Fig. 91 is shown a ceiling with porous tile a carried on T bars b. The under side of the bars is protected by plaster c, while the top of the web is enveloped by cement mortar d. The tiles are from 2 to 3 inches thick and from 16 to 24 inches long, weighing from 11 to 15 pounds per square foot, not including plastering.
Fig. 92 shows a ceiling made of hollow tile, laid on T bars, with plates to protect the flanges.