210. Steel, in the shape of wire netting and of expanded and perforated metal lath, has come into very extensive use for the support of plastering, especially in fireproof structures. Plaster becomes more firmly attached to metal than to wood, and will not become loosened by ordinary accidents; and, as the steel is more or less completely embedded in the plaster, the latter protects it effectively from fire, whereas wood lath is only partially covered. Apart from the fireproof qualities, metal lath are of value in wood construction, from the fact that plaster laid on them will not crack nor fall off, from shrinkage in the woodwork; and, if the lath are set away from the joists and studs, these will not show through the plaster, as is generally the case when it is laid on wooden laths.

Probably the best fireproof lath consists of strong wire cloth stretched tightly over metal furrings. In the difficulty of doing this properly lies one of the objections to the use of wire cloth, as, unless the netting is quite rigid, it will yield considerably as the coats of plaster are applied; however, manufacturers now furnish stretchers, the use of which, to a great extent, obviates this difficulty. Objection has also been raised that both wire and expanded metal lath require much more plaster than wooden lathing. In this fact lies their great value, for, as the mortar is the fireproofing material, the lathing should be completely embedded in it; otherwise, thin metal is of little, if any, greater value than wooden lathing. Numerous severe tests, both experimental and in actual fires, have demonstrated that plaster - especially the hard kinds - applied to wire cloth, will successfully protect woodwork from fire, if the wood be completely covered by it.

Wire lathing may be had in numerous sizes; the most used size has about 2 1/2 X 2 1/2 meshes per square inch, and is made of No. 20 gauge wire. It comes in rollsfrom 32 to 36 inches wide, but some manufacturers supply it in widths up to 8 feet. The lathing may be had either plain, painted, or galvanized. The latter form, while more expensive, is preferable, as it is much stiffer and less liable to rust than the plain cloth. Painted lath is nearly as good as the galvanized, and costs considerably less.

211. Furring

Furring. When wire lath and plaster are used to protect woodwork, they should not be in contact with it, but separated by a space of at least 1/4 inch, while more is advantageous. To obtain this spacing, furring strips are necessary. These may be either woven into the wire cloth, or be separate pieces, fastened to the wood before the lathing proper is applied. In the latter case, the strips consist of corrugated or flat iron, of proper width to give the required air space when set on edge - usually about 1/2 or 3/4 inch - and are secured to the wood by small staples. The strips generally run lengthwise of the floorbeams and studding; if laid crosswise, they have not such a firm bearing, and should be placed closer together, or heavier strips should be used.

A substitute for this kind of furring is made of 1/8-inch rods, spaced from 6 to 8 inches apart, and kept away from the wood by separators made of small pieces of thin metal, having the ends turned up from 1/2 to 1 inch at right angles to the back, which is slotted to permit the insertion of staples. This furring should run across the joists, or studs, and the rods may be placed as close together as required.