Metal Lath
A metal base often is used in plaster construction because it imparts strength and resists cracking. Plaster holds to metal lath by mechanical bond between the initial coat of plaster and the metal. So it is important that the plaster completely surrounds and embeds the metal.
Basic types commonly used are expanded-metal, punched sheet-metal, and paper-backed welded-wire lath. Woven-wire lath may be used as a supplemental reinforcement over solid plaster bases, but not as the primary base for gypsum plaster. Wire lath should be made of galvanized, copper-bearing steel. The other types should be made of galvanized steel or copper-bearing steel with a protective coat of paint.
Expanded-metal lath is fabricated by slitting sheet steel and expanding it to form a mesh. Several types are available:
Diamond-mesh lath, with more than 11,000 meshes/yd2, is an all-purpose lath, suitable as a base for flat or curved plaster surfaces (Fig. 11.21). The small meshes are helpful in reducing droppings of plaster during plastering. A self-furring type also is available. When it is attached to a backing, it is separated from the backing by at least 1â„4 in. Thus, self-furring lath is convenient for use as exterior stucco bases and bases for column fireproofing and replastering over old surfaces.
Flat-rib expanded-metal lath comes with smaller openings than diamond mesh, and it has ribs parallel to the length of the sheet that make it more rigid. Flat-rib lath is generally preferred for nailing to wood framing and tying to framing for flat ceilings, but it is not suitable for contour lathing.
High-rib expanded-metal lath is used when greater rigidity is desired, for example, for spacing supports up to 24 in c to c and for solid, studless plaster partitions.
The lath has a herringbone mesh pattern and V-shaped ribs running the length of each sheet. For 3â„8-in rib lath, 3â„8-in-deep ribs are spaced 41â„2 in c to c, alternating with inverted 3â„16-in ribs. For 3â„4-in rib lath, 3â„4-in-deep ribs are spaced 6 in c to c. This type of high-rib lath may be used as a form and reinforcement for concrete slabs, but its thickness makes it generally unsuitable for plaster construction.
The 3â„8-in rib lath may also be used as a concrete form, but its rigidity makes it unsuitable for contour lathing.
Welded-wire lath should be made of wire 16 ga or thicker, forming 2 2-in or smaller meshes, stiffened continuously parallel to the long dimension of the sheet at intervals not exceeding 6 in. The paper backing should comply with Federal Specifications UU-B-790, Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellant, and Fire Resistant). Acting as a base to which plaster can adhere while hardening about the wire, the backing should permit full embedment in at least 1â„8 in of plaster of more than half the total length and weight of the wires.
Table 11.9 lists limiting spans for various types and weights of metal lath for ceilings and walls.
Tying and Nailing. Installation of metal lath should meet the requirements of ASTM C841. Attachments of metal lath to supports should not be spaced farther apart than 6 in along the supports.
When metal framing or furring is used, metal lath should be tied to it with 18- ga, or heavier, galvanized soft-annealed wire. Rib lath, however, should be attached to open-web steel joists with single loops of 16-ga, or heavier, wire, or double loops of 18-ga wire, with the ends of each loop twisted together. Also, rib lath should be tied to concrete joists with loops of 14-ga, or heavier, wire or with wire hangers not less than 10 ga.
With wood supports, diamond-mesh, flat-rib, and welded-wire lath should be attached to horizontal framing with 11â„2-in, 11 ga, 7â„16-in-head, barbed, galvanized, or blued roofing nails, driven full length. For vertical wood supports the following may be used: 4d common nails; 1-in, 14-ga wire staples driven full length; and 1-in roofing nails driven at least 3â„4 in into the supports. Common nails should be bent over to engage a rib or at least three strands of lath. Alternatives of equal strength also may be used.
Metal lath should be applied with long sides of sheets spanning supports.
Each sheet should underlap or overlap adjoining sheets on both sides and ends.
Expanded-metal and sheet-metal lath should be lapped 1â„2 in along the sides, or have edge ribs nested, and 1 in along the ends. Welded-wire lath should be lapped one mesh at sides and ends. All side laps of metal lath should be fastened to supports and tied between supports at intervals not exceeding 9 in.
Wherever possible, end laps should be staggered, and the ends should be placed at and fastened to framing. If end laps fall between supports, the adjoining ends should be laced or securely tied with 18-gage, galvanized, annealed steel wire.
Normally, metal lath should be applied first to ceilings. Flexible sheets may be carried down 6 in on walls and partitions. As an alternative, preferable for more rigid sheets, sides and ends of the lath may be butted into horizontal reentrant angles and the corner reinforced with cornerite. But for large ceilings (length exceeding 60 ft in any direction, or more than 2400 ft2 in area) and other cases in which restraint should be avoided, and for portland-cement plaster ceilings, cornerite should not be used. Instead, the abutting sides and ends should terminate at a casing bead, control joint, or similar device that will isolate the ceiling lath and plaster from the walls and partitions.
Similar considerations govern installation of metal lath at vertical reentrant corners.
Between partitions, flexible sheets may be bent around vertical corners and attached at least one support away from them; more rigid sheets may be butted and the corner reinforced with cornerite. But where restraint is undesirable at reentrant corners, for example, where partitions meet structural walls or columns, or where load-bearing walls intersect, cornerite should not be used. Instead, the walls, partitions, and structural members should be isolated from each other, as described for ceiling-wall corners.
