357. Wherever lathing is required in buildings that are intended to be thoroughly fireproof, only stiffened wire or expanded metal lath should be used. If one of the hard plasters are to be used, close-warp (2½x5 mesh) should be specified, and the lathing should be either painted or galvanized. (See Section 333.)
In buildings having hollow tile floor construction but very little, if any, lathing is used, as all the walls, ceilings and partitions are of tile, on which the plastering is directly applied. For such buildings either machine-made lime mortar (such as is described in Section 340) or one of the hard plasters should be used.
The method of forming a beam and cornice in this way is shown by Fig. 234. The general profile is formed by bending light iron by hand on a shaping plate to the desired outline. These are secured in position and longitudinal rods fastened to their angles, after which the wire lathing is applied.
Fig. 235 shows the manner of furring steel and iron columns when protected by wire lath and plaster.
Such grounds are more firm to carry the heavy stucco, and the plastering is not as liable to be broken by streams of water in case of a fire.
They are, therefore, generally preferred to metal grounds, and are used almost entirely in the U. S. Government buildings when the ceilings are of tile.
The various pieces forming the ground should be bolted to the floor construction with ¼-inch T-head bolts spaced not over 12 inches apart longitudinally, and at least two bolts to each piece.
These terra cotta grounds are usually made by manufacturers of flue linings and pipes, as their machinery is better adapted for the pur-pose than that used for making fireproof tile. 358. Thin Partitions of Metal Lath and Studding. - As stated in Section 320, partitions only 2 inches thick are now quite extensively used in office buildings and hotels to economize floor space. Most of these partitions are constructed of upright studding of ¾-inch channel bars spaced from 12 to 16 inches on centres and fastened securely to the floor and ceiling. On one side of this studding, metal lathing, preferably of stiffened wire cloth, is stretched and securely laced to the studs. The partition is then plastered on both sides with hard plastering and finished in the usual manner. If properly executed the partition will be stiff enough to answer all the purposes for which it is required, and is, of course, absolutely fireproof. Only the best of hard wall plasters should be used for such partitions, however, as the stiffness of the partition depends very much upon the solidity of the plastering; hence the firmer and harder the plastering the more substantial will be the walls. By using 2-inch channels and lathing both sides a very stiff partition is obtained, but, of course, at greater expense. The New Jersey Wire Cloth Co. makes a special lathing for thin partitions, which has a ¼-inch solid rod woven in at intervals of 7½ inches. The lath is stretched over the studs so that the rods cross them at right angles. The lath, after being tightly stretched, is laced to the studs at every point where the rods cross them. Expanded metal or sheet iron lathing may be fastened to the studding in place of the wire lath if preferred (see Section 388), but the wire lathing makes the stiffest partition, from the fact that it can be tightly stretched.
If wire lath of the standard mesh is used some provision must be made for securing the wooden base and picture mould.
Fig. 237 shows the method ordinarily adopted for securing the base. For securing the picture mould, strips of wood may be laced to the lath at the required height before plastering.
When close-warp lathing, plastered with hard mortar, is used, No. 14 or 16 screws will engage in the meshes of the wirework, and all woodwork can be fastened directly to the partition with wood screws.
Partitions of thin porous tiling were described in Chapter IX (Fireproofing)., Section 320.