thumb|right|250px|Sawn lath, seen from the back with hardened [[plaster from the other side showing through. Sawn lath became popular after the introduction of the circular saw in the 19th century.]]
thumb|Roof laths span across between the rafters and support the wood shingles.A lath or slat is a thin, narrow strip of straight-grained wood used under roof shingles or tiles, on lath and plaster walls and ceilings to hold plaster, and in lattice and trellis work.
Lath has expanded to mean any type of backing material for plaster. This includes metal wire mesh or expanded metal that is applied to a wood or metal framework as matrix over which stucco or plaster is applied, as well as drywall products called gypsum or rock lath. Historically, reed mat was also used as a lath material.
One of the key elements of lath, whether wooden slats or wire mesh, are the openings or gaps that allow plaster or stucco to ooze behind and form a mechanical bond to the lath. This is not necessary for gypsum lath, which relies on a chemical bond.
Types of lath
thumb|[[Wood splitting|Riven lath, each piece has been split from a log]]
Wooden
Wooden-slat laths are still used today in building construction to form a base or groundwork for plaster, but modern lath and plaster applications are mostly limited to conservation projects.
Tiles, slates, and other coverings on roofs and walls are often fastened to laths, sometimes also called battens or slats. Such strips of wood are also employed to form lattice-work, or are used as the bars of Venetian blinds, and window shutters. Lath is also used on many tobacco farms in the Connecticut Valley as a means to carry and hang the plant in barns. This is achieved by using one of two methods: hooking or spearing. A "spear" lath is a regular lath that is held in an upright position. A worker then mounts a spear on top and "spears" the tobacco onto the lath. The other form of lath is called "hook" lath, which has small hooks attached that allows workers to hook the stems of tobacco plants onto the lath, often between two lengths of twine attached to the lath and twisted mechanically.
A lathhouse is an open structure roofed with laths in order to grow plants which need shelter from the sun.
Laths were also used to fix reeds to a timber structure before plastering.
In Cape Cod, laths were used in the early 1880s for building wooden lobster traps.
Historically there were three ways of making wood lath for plaster: riven lath, accordion lath, and circular sawn lath. Riven lath was traditionally split with the grain from chestnut, oak, and similar hardwoods, or from softwoods like eastern white pine. Individual laths were riven and nailed in place. Because they are split with the grain, riven lath is stronger than later forms of lath production. Accordion laths are thin, sawn boards that are partially split with a hatchet or axe. The splits are then spread apart to form gaps for the plaster to key into. The name derived from the spreading action, which is like pulling an accordion open. After the circular saw came into use in the early 19th century, lath for plastering was sawn in sawmills and delivered to the building site. Ihe 1930s to 1940s the use of lath and plaster was replaced with plasterboard, which was cheaper and easier to use.
Counter-lath
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Counter-lath is a term used in roofing and plastering for a piece of wood placed perpendicular to the lath. In roofing, a counter-lath is a slight piece of timber parallel with and between common rafters to give the lath extra support, or a lath placed by eye between every two gauged ones. When plastering, sometimes a counter-lath is placed perpendicular to the lath as a fillet (a thin, narrow strip of material) to space the lath off of the surface to allow the plaster to pass through the lath and create a key.
Several types of metal lath have been developed for a variety of applications:
- Expanded metal lath is made by slitting and pulling apart a thin sheet of metal, which produces diamond-shape holes through which the plaster can form keys. and multiple variations were developed in the 1930s. Gypsum lath is available with a foil facing, which acts as a vapor barrier and heat reflector, and as a veneer base for plaster veneer.
Framing
Lath can be attached directly to the frame of a building, such as the studs of a timber structure. Alternatively, lath can be attached to a timber or metal frame called a furring, which is then attached to the building structure. Furrings are often used in masonry construction. Frames are also used when using lath and plaster to create decorative, curved, or ornamental work.
Lath failure
thumb|Lath with sections of failing and failed sections
There are several reasons that a plaster and lath wall may fail. First, the lath itself can sometimes pull away from the frame on which it is mounted. This is generally due to the use of non-galvanized nails. The lath can also fail because of decay from moisture or insect damage. Moisture can also cause wooden lath to expand and contract, causing the plaster around it to crack.
See also
- Garden trellis
- Lath art
- Lattice truss bridge