This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
207. In old work, the lath was generally made of oak, but in current practice, pine, spruce, and hemlock are used. The regular size of the lath strips is 1/4 in. X 1 1/2 in., and 4 feet in length; this length regulates the spacing of the furring strips, studs, and joists in order to prevent waste of material.
The lath may be either split or sawed; the former gives a better wall, as there are no cross-grained fibers to reduce its strength; while the presence of the cross-grained fibers in sawed lath makes it liable to curl and warp, due to the absorption of moisture from the mortar, still, being somewhat cheaper, it is more generally used in this country. To avoid the warping already referred to, the lath should be straight grained; to insure durability, it should be well seasoned, free from any appearance of sap, rot, or incipient decay; for strength, it should be clear of shakes and large or loose knots; and to prevent the subsequent discoloration of the plaster, the lath should be free from live knots and resinous pockets.
Laths are sold by the thousand, which will cover about 570 square feet, estimating 1 3/4 laths to the square foot. A fairly good workman will put on from 1,000 to 1,200 laths per day of 10 hours.
208. The laths are nailed in place in parallel rows, the edges being kept a full 1/4 inch apart, to enable the soft plaster to be pressed through and form the key. The ends of the laths should not lap over each other, but should be butt jointed and flush. Continuous joints should not occur on one support, but the lathed surface should be divided into panels from 15 to 18 inches in width, and the joints be made to break on alternate supports, as shown by the panel a b d c, Fig. 104; otherwise continuous cracks will be liable to form. The laths are usually attached to joists and studs with cut or wire nails, about 1 1/8 inches in length, and having large, flat heads, one nail being used at each support; more solid work is obtained where two nails are used at each support, but in any case there should be at least two nails in each lath at each end. The nails should be galvanized, to prevent the moisture from readily attacking the iron and causing the nails to rust, which will produce large yellow patches on the surface of the plaster. Where certain plasters, such as Keene's cement, are applied, copper nails must be used, as the plasters contain ingredients that will cause iron nails to rust rapidly.
Where joists or studs are over 2 inches in width, a strip should be attached to them so as to allow the lath to be kept clear of the wide surface and enable the plaster to form the key. Nails are sometimes thickly studded over wide surfaces of wood, such as beams, the plaster rivet being formed around the heads of the nails; but, where practicable, lath splits nailed at intervals to the beam, transversely to the lathing, will give better results. The practice of closing angles and narrow spaces with the laths placed vertically, should not be allowed, as an imperfect key is formed, owing to the weight of the mortar causing a downward flow of its substance.
Sheathing Lath. A combined sheathing and lath has been used to some extent, and, while somewhat more costly, is an excellent substitute for ordinary lath. It consists of 7/8-inch boards, 8 inches in width, cut in uniform lengths, and grooved as shown in Fig. 99. Such sheathing is especially valuable when the outer walls of wooden buildings are to be back plastered; that is, the sheathing, grooved side inwards, is nailed to the outside of the studs, the plaster being applied between the latter. This method makes a very warm house, but, when a second thickness of plaster is used inside the studs, it is, of course, more expensive than the ordinary plastering. This lath is also used for inside work, and has the advantage of requiring less mortar than the common lathing, and also that nails can be driven anywhere in them without danger of loosening the plaster.