332. Brick walls and hollow tile ceilings and partititions do not require lathing, as the plastering may be applied to them directly, the brick and tiles having an affinity for the mortar which holds it securely in place. All other constructions require some form of lathing to serve as a ground to receive and hold the plaster.
Wooden Laths. - Practically all dwellings of moderate cost, and a large proportion of other buildings, are still lathed with wooden laths, and if of good quality they give very satisfactory results where no fireproof quality is expected. It is generally admitted that the best wood for laths is white pine, although nearly as many are made of spruce, which answers very well. Hard pine is not a good material for laths, as it contains too much pitch.
Wooden laths should be well seasoned and free from sap, bark and dead knots. Small sound knots are not particularly objectionable. Bark is often found on the edges of laths, and is probably the greatest defect that they are subject to, as it is quite sure to stain through the plaster.
The usual dimensions of wooden laths are ¼ x 1¼ inches in section and 4 feet long; the width and thickness vary somewhat in different mills, but the length is always the same. The studding or furring strips should therefore be spaced either 12 or 16 inches apart from centres; 12-inch spacing gives five nailings to the lath, and 16-inch spacing four nailings. The former obviously makes the stronger and better wall. It is particularly desirable that laths on ceilings have five nailings, as there is more strain on them than on those on the walls.
Sheathing Lath. - Some years ago a combined sheathing and lath, known as the Byrkit-Hall Sheathing Lath, was placed on the market. This lath is made of 7/8-inch boards, 8 inches wide and of regular lengths, grooved as shown in Fig. 229. To what extent it has been used the author is not informed, but it would appear to be an excellent material for sheathing the outside walls of wooden buildings that are to be back plastered. Such sheathing plastered on the inside will make a very warm house, and effect much saving in the cost of heating. It is also claimed that this lath is superior to the wooden lath for inside work, although one would naturally suppose that the shrinkage of the wood would cause the plastering to crack. It certainly makes a saving in the amount of mortar used, and also has the advantage that a nail can be driven into it anywhere without spoiling or loosening the plastering. A special lath is made for hard plasters, adamant, etc.
Fig. 229. - Byrkit-Hall Sheathing Lath.
About eighteen years ago, when the interest in fireproof construction became more general, wire netting came into use as a substitute for the wood lath. It was found that the strands of the netting became completely imbedded in the plaster and held it so securely that it could not become detached by any ordinary accidents. The plaster also protects the wire from the heat, and the body of the metal is so small that there is no appreciable expansion of the metal when subjected to fire.
The author believes that heavy wire cloth tightly stretched over metal furrings forms the most fireproof lath now on the market, and he has personally seen it demonstrated by severe experimental tests, and by actual fires in buildings, that plaster on wire cloth, and particularly hard plasters, will protect the woodwork from a severe fire so long as the plaster remains intact, provided there are no cracks or loopholes at the corners and around columns where the fire can get through.
The objection has been found to the ordinary wire lath that it is difficult to stretch it so tight that it will not yield to the pressure exerted in applying the several coats. Another objection that is made to the wire lath, and also to the expanded lath (Fig. 231), is that they take a great deal of plaster. From the standpoint of first cost this is undoubtedly a valid objection, but from a fireproof standpoint the great amount of mortar used is its principal value. It should be remembered that the mortar is the fireproof part of the wall or ceiling, and not the metal. No metallic lath, the author believes, should be considered as fireproof which does not, in use, become imbedded in the mortar, for if the thin coating of plaster peels off the metal lath will resist the fire no better than the wood lath, and will be more in the way of the fireman.
Wire lathing is now made in great variety to meet the requirements of the different plastering compositions and the varying conditions of construction.
Plain lathing is plain* wire cloth, usually 2½ x 2½ meshes to the inch, made from No. 17 to No. 20 wire. No. 20 is more generally used than any other size.
* The word plain is here used to designate ordinary wire cloth, without corrugations or stiffening bars. As used by the trade, the word "plain" means lathing that is not painted or galvanized.
The lathing is also sold plain, painted and galvanized. Painted or galvanized lathing should be used in connection with special hard plaster compounds. Painted lathing costs about one cent per square yard more than "bright" lathing.
Galvanizing the wire cloth after it is woven adds very much to its stiffness, as the zinc solders the wires together where they cross. Galvanized lathing is also less liable to corrosion before the plastering is applied than the plain lathing.
The usual widths of wire lathing are 32 and 36 inches, although the Roebling lath may be obtained of any width up to 8 feet.
All wire lathing should be stretched tight when applied, so as to insure a firm surface for plastering. For this purpose stretchers are supplied by the manufacturers.
Furring for Wire Lath. - In order to properly protect wooden construction, such as beams, posts, studding or plank, from fire, by wire lath and plaster, it is essential that the lath be kept at least 3/8 inch away from the woodwork by iron furring of some form, and a 1-inch space is much better. This setting off of the lath from the wood is generally done either by means of bars woven into or attached to the lathing, or by means of iron furring put up before the lathing. Probably the most common method of furring with iron for wire lath has been by means of band iron, either straight or corrugated, ½ inch or ¾ inch wide, set on edge and secured to the under side of the joist or plank by narrow staples, driven so as to keep the iron in a vertical position.
On floor beams and studding, unless heavy iron is used, it is necessary to run the furring lengthways of the beams and studding, and, as the latter are seldom less than 12 inches on centres, this does not give close enough bearings to secure a stiff surface for the plastering.
Under plank (mill) floors the band iron should be spaced every 8 inches, and, if corrugated iron is used, a very satisfactory surface is obtained. After the furring is fixed in place the cloth is then stretched over it and secured by staples nailed over the wire and the band iron.
Hammond's Metal Furring. - A much better system of furring, and, so far as the author is informed, the most perfect of all systems of separate furring over woodwork, is that known as the "Hammond" furring, and shown by Fig. 230. It consists of a combination of sheet metal bearings and steel rods. The rods form the furring for keeping the wire cloth away from the timber, and the bearings form the offset for the rods, both being secured to the joist, studding or plank by means of staples, as shown in the figure. The rods, being only about ½ inch in diameter, become completely imbedded in the plaster when it is applied, and as the plaster hardens it unites the rod and cloth so as to make a much more rigid surface than is possible where band iron furring is used. The rods also may, and in fact should be, run across the beams or studding, and may therefore be spaced as close together as desired. It is recommended that the spacing of the rods be made 7 1/5 inches where the joist are 12 inches on centres and 6 inches when the joist are 16 inches on centres (being 5 and 6 bars to each strip of lathing). The bearings are ½ inch and 1 inch deep, the latter being recommended, as they give a greater air space between the plaster and timber, which is especially desirable in lathing around solid timbers or under planking. The rods come in lengths of about 10 feet.
This system of furring is applicable to wooden posts, partitions and any form of wood construction; it is readily put up, and is but little more expensive than band iron. After the furring is in place the wire cloth (which should be No. 20 gauge, and painted or galvanized if hard plasters are to be used) is stretched over it, preferably in the same direction as the rods, and secured by staples driven over the wire and one side of the bearing, as shown in the figure.
Corrugated Wire Lathing. - A lathing made of flat sheets of double twist warp lath, with corrugations 3/8 of an inch deep running lengthwise of the sheet at intervals of 6 inches, has been used to some extent. The sheets are made 8x3 feet in size and applied directly to the under side of the floor timbers, to partitions or to brick walls, and fastened with staples. The corrugations afford space for the mortar to clinch behind the lath, and thus do away with the necessity for furring strips; they also strengthen the lathing.
Stiffened Wire Lathing. - In order to avoid the labor and expense of furring with metal, wire lathing having the furring strips attached to the fabric was introduced some years ago, and has been very extensively used, and the author would recommend that whenever wire lathing is used over wood construction that either one of the stiffened wire laths, or ordinary wire cloth with the Hammond furring, be specified.
Two varieties of stiffened wire lathing are now on the market. Each has been extensively used, with satisfactory results.
The Clinton stiffened lath has corrugated steel furring strips attached every 8 inches crosswise of the fabric by means of metal clips. These strips constitute the furring, and the lath is applied directly to the under side of the floor joist, or to planking, furring, brick walls, etc. This lath is made in 32-inch and 36-inch widths and comes in 100-yard rolls.
The Roebling stiffened lathing, made by the New Jersey Wire Cloth Co., is made of plain wire cloth, in which, at intervals of 7½ inches, stiffening ribs are woven. These ribs have a V-shaped section and are made of No. 24 sheet iron, and vary from 3/8 to 1½ inches in depth. The 3/8-inch rib is the standard size for lathing on woodwork. This lathing requires no furring, and is applied directly to woodwork or walls with steel nails driven through the bottom of the V, as shown in Fig. 230 A.
The No. 20 V-rib stiffened lathing affords a satisfactory surface for plastering, when attached to studs or beams spaced 16 inches apart. The lathing should be applied so that the widths will join on a beam or stud.
The 1¼-inch V-rib lathing is used for furring exterior walls. It provides an air space between the wall and plaster.
For iron construction a ¼-inch solid steel rod is substituted for the V-rib, and the lathing is attached to light iron furring with lacing wire. The Roebling lath is made with 2½x2½, 3x3 and 3x5 mesh, the latter being known as "close-warp." The 2½x2½ mesh should be used for ordinary lime and hair mortar, and the 3x3 or 3x5 mesh for hard plasters and thin partitions. This lathing is also sold bright, painted and galvanized.
The No. 20 painted wire has been extensively used, and much of it has been in service for from 6 to 8 years and is now apparently as good and strong as ever, so that there appears to be no necessity in ordinary work of using heavier wire or galvanized netting.
Fig. 230 A.
The galvanized wire is stiffer than the painted, and would possibly wear longer, but it is doubtful if the advantages are at all proportionate with the cost.