In office buildings having a flat roof there is generally an air space or attic between the roof and ceiling of the upper story, varying from 3 to 5 feet in height. This space is often utilized for running pipes, wires, etc. Buildings having pitched roofs necessarily require a ceiling below to give a proper finish to the rooms in the upper story and to make the rooms comfortable. In office buildings the ceiling under the roof is generally of a similar construction to that of the floors, although when systems like the Roebling, Columbian or Metropolitan are used in the building only the suspended ceiling plate is required between the beams, and the latter may be made very light.
Under pitch roofs (and sometimes under flat roofs) a suspended ceiling is generally used. T-bars (usually 3x3 inches in size) are hung from the roof construction by means of light rods, and the ceiling constructed either by means of wire or expanded metal lathing laced to light angles, or flat bars placed between the T's, or by thin tiles of dense or porous terra cotta. If a tile ceiling is to be used, the author believes that porous or semi-porous terra cotta should be given the preference. Whichever material is used, the shape of the tiles should be such that they will drop below the flanges of the T's, so as to protect the metal.
Fig. 196 shows the usual section of porous ceiling tile, and Fig. 197 an improved shape of semi-porous ceiling tile made by the Pioneer Co. The width of the porous tile is 16 inches for 2-inch tile and 18, 20 and 24 inches for 3-inch tile. The 2-inch tiles weigh 11 pounds and the 3-inch tiles 15 pounds per square foot, exclusive of the plastering. The tiles shown in Fig. 197 are 3 inches thick and weigh 14½ pounds per square foot.
Suspended ceilings of wire lath and plaster weigh only about 12 pounds per square foot, including the plastering.
The columns and girders are more exposed to intense heat than the floor beams, and should be protected in the most efficient manner possible, as any expansion in the columns or girders would have a most disastrous effect.
Columns and girders are also more exposed to the streams of water, which tend to dislodge or break through the casing. As a rule, the manner in which these portions of the structural work are protected depends largely upon the system of floor construction employed. Naturally the parties having the contract for the fireproofing of the floors generally wish to use their system, or materials, for protecting the columns and girders, and thus the question of cost often works to the disadvantage of the better system.
In buildings with tile filling between the floor beams the columns and girders are usually cased with tiles; when one of the concrete systems of floor construction is adopted, the same material is, as a rule, employed for protecting these members, although there is no necessity for using the same material in both cases.
The unbiased opinion of architectural engineers and those who have made a study of fire protection is, the author believes, in favor of solid porous tiling for girder and column casings. The author believes that the best possible protection for these members will be obtained by using solid blocks of porous terra cotta, well secured to the metal, and then covered with wire or expanded metal lathing plastered with hard mortar, such as "Acme" or "King's Windsor," the metal lathing serving principally as a protection from the blocks becoming dislodged.
Girders. - The usual forms of dense or porous tile casings for girders are shown in Fig. 198. Shapes very similar to these are made by all the manufacturers of both dense and porous tiling. Casings of dense tiling should preferably be made hollow, thus giving a second air space, as shown in Fig. 195, B. Methods of casing girders with metal lathing will be shown in Chapter XL