The under side of flat tile arches is usually finished with two coats of plaster applied directly to the bottom of the tiles. If there are inequalities in the surfaces of the arches they should be filled with natural cement and sand mortar before plastering. False plaster beams may either be formed on metal furring, bolted to the under side of the arches and covered with wire lathing, or the furring may be of wood, as its consumption in case of fire would in no way endanger the building. Metal furring, however, is better, as it does not shrink.
The general custom in regard to the size of floor strips and depth of filling is to use 2X4-inch well-seasoned wood strips, beveled to 2 inches wide on top and laid at right angles to the beams and 16 inches apart from centres. The concrete is first leveled to the tops of the highest beams and the strips then laid in place by the carpenter. The mason then fills between the strips to within \ inch of their top with concrete, pressed down hard against the strips. A single matched flooring is then nailed to the wood strips. In New York 3 X 4-inch strips are often used, the strips being notched down over the beams 1 inch. The strips, also, do not always run at right angles to the beams, although the general opinion appears to be that they should do so wherever practicable.
The general custom amongst Chicago architects is to allow 3½-inches from the top of the beams to the top of the finished floor. This gives a sufficient space between the beams and flooring for running gas pipes or water pipes, as shown in Fig. 183. Wherever buildings are piped for gas, and especially office buildings, it is absolutely necessary to leave sufficient space between the tops of the steel beams and the bottom of the flooring for running branches to centre outlets.
Wherever the nailing strips cross the floor beams or girders they should be fastened to them by means of iron clamps, made so that one end can be hooked over the flange of the steel beam and the other end driven into the side of the wood strip. When the strips run parallel with the beams it is good practice to nail pieces of hoop iron across the under side of the strips about 4 feet apart, to hold the strips more firmly in place, as the concrete alone does not hold them with sufficient firmness. The hoop iron strips should be 1½ x 1/8 inch and 10 inches long, and should be secured by two clout nails.
The concrete used for the filling on top of the arches and between the nailing strips should be made of screened boiler cinders, mixed with lime mortar gauged with plaster of Paris or Portland cement, the cinders being used on account of their lightness. The concrete must become thoroughly dry before the flooring is laid. As this requires considerable time, dry cinders without any lime or cement has been used in a few office buildings where it was necessary to rush their completion. The best architects, however do not recommend the use of dry cinders when it can be avoided.
Occasionally, where the beams are of unusually long span, a 10-inch or 12-inch arch is set between 15 or 20-inch beams. In such cases it is better to fill in on top of the arches with partition tile or -shaped tile made for the purpose.
If the floors are to be tiled the concrete between the bottom of the tiles and the top of the arch should be made of Portland cement, sand and crushed stone.
Wooden floors should be laid continuously over the entire area to be covered, without reference to partitions, where the same are liable to be changed to suit tenants. Permanent partitions should be erected before the floors are laid.
Fig. 183 shows the floor construction used in the "Fair" Building, Chicago, Jenney & Mundie, architects, and also the fireproofing of the columns. This cut is also typical of many other buildings recently erected in Chicago.