This lath (Fig. 231), now probably well known to architects, is made from strips of thin, soft and tough steel by a mechanical process which pushes out or expands the metal into oblong meshes, and at the same time reverses the direction of the edge, so that the flat surface of the cut strand is at right angles with the general surface of the sheet.
Two sizes of meshes are made, 3/16x1¼ inches and ¼x1¼ inches, the former being best adapted for the hard mortars and the latter for lime mortar. Both kinds are made in sheets 8 feet long and from 14 to 20 inches in width, 18 inches being the standard width.
This lath being flat and of considerable stiffness does not require to be stretched, and can be fastened directly to the under side of floor joist or to wood studding. If used on plank it should be fastened over metal furring strips. When applied to studding the lath should be placed so that the long way of the mesh will be at right angles to the studding, as shown in Fig. 231, as this insures the greatest rigidity. The studding or furring strips should be spaced 12 inches on centres and the lathing secured with staples 1 inch long, driven about 5 inches apart on the stud or joist. The lath, when applied, is a scant ¼ inch thick, and to obtain a good wall ½-inch grounds should be used.
There are several companies manufacturing this lathing under territorial rights, and it has been extensively used with very satisfactory results. The author believes it to be the most fireproof lath made from sheet metal.
There are some six or more styles of metal lath made from sheet iron or steel by perforating the sheets so as to give a clinch to the mortar. The sheets are generally corrugated or ribbed, also, in order to stiffen them and keep them away from the wood. There is not a great difference between these laths, although some styles may possess certain advantages over the others.
In general, the author would prefer those styles which have the greatest amount of perforations, or which approach the nearest to the expanded lath. All of these laths come in flat sheets about 8 feet long and 15 to 24 inches in width, and are readily applied to woodwork by means of barbed wire nails. The nails should be driven every 3 inches in each bearing, commencing at the centre of the sheet and working toward the ends. These lath work very nicely in forming round corners and coves, and are generally preferred to the wire lath by plasterers, as they are easier to put on. They are certainly much superior to wood laths. Metal lath should never be cut at the angles of a room, but bent to the shape of the angle and continued to the next stud beyond. This strengthens the wall and prevents cracks at the angles.
Of the various forms of sheet metal lath in common use, the Bostwick lath (Fig. 231 A) is perhaps the best known and most extensively used. It is made of sheet steel, with ribs every \ of an inch in the width of the sheet, and loops, 3/8x1¾ inches, punched out between the ribs; the lath should be applied with the loop side out. This lath can be put on as fast as the wood lath, and is especially well adapted to round corners and coves.
When using common lime mortar on metal lath the first coat should be gauged with plaster of Paris. Either painted, galvanized or japanned lath should always be used for hard plasters made by a chemical process, such as King's Windsor and adamant.
Aside from their fireproof qualities, wire or metal laths possess the advantages that plastering applied to them will not crack from shrinkage in the woodwork, nor can the plaster fall off. If the lathing is set away from the wood studding, the location of the timbers will not be shown by the plaster, as is invariably the case after a few years when wood laths are used. Metal laths are also proof against rats and mice, which makes them especially desirable in certain kinds of store buildings. Nearly all these advantages are lost when unstiffened wire cloth is stretched over wood furrings.
Fig. 231 A.