Lath

A lath is a thin, narrow strip of material used as a base for various surfaces.

While many people think of lath as only wooden slats, the term now includes any backing material for plaster. This includes metal wire mesh, expanded metal, or even gypsum products known as rock lath. Lath serves many different purposes in construction and agriculture. It can support roof shingles or tiles on a roof. It can also be used to create garden trellises, window shutters, or Venetian blind bars.

In some places, like the Connecticut Valley, tobacco farmers use lath to hang plants in barns.

To understand how lath works, you must understand a process called keying. This is how plaster or stucco stays attached to a surface. For wooden or metal lath, the material must create a mechanical bond. When a builder applies wet plaster, it oozes through the gaps or holes in the lath. The plaster travels to the back side of the material. Once the plaster hardens, it forms a shape known as a key. These keys act like tiny hooks that lock the plaster to the lath.

Without these keys, the plaster would not have the strength to stay on the wall.

Historically, there were three main ways to produce wooden lath. The oldest method produced riven lath. This was made by splitting hardwoods like oak or chestnut, or softwoods like pine, along the grain. Because it was split with the grain, riven lath was very strong. Another method was accordion lath. These were thin, sawn boards that were partially split with an axe. The splits were spread apart to create gaps, much like pulling an accordion open. In the early 19th century, the circular saw changed production. Lath could then be sawn in sawmills and delivered directly to building sites.

Modern construction uses several specialized types of lath. Metal lath became common in the late 19th century and is used extensively today. Expanded metal lath is made by slitting and pulling apart a thin metal sheet to create diamond-shaped holes. Ribbed lath features V-shaped ribs to add stiffness for spanning larger distances. There is also self-furring lath, which is dimpled to hold it away from a surface. For corners, builders use pre-bent corner lath or a wire mesh called Cornerite.

These metal options provide both a matrix for the stucco and added rigidity to the structure.

Another important modern material is gypsum lath, also called rock lath. This consists of gypsum plaster sandwiched between two sheets of absorbent paper. Unlike wood or metal, gypsum lath does not rely on mechanical keys to stay attached. Instead, the finish side is treated with gypsum crystals so the plaster can form a chemical bond. It is often used on straight walls because the material is very rigid. Gypsum lath is also noncombustible, which makes it a safer choice for many buildings. Some versions even include a foil facing to act as a heat reflector or vapor barrier.

Even though lath is very useful, these walls can sometimes fail. One reason is the use of non-galvanized nails, which can cause the lath to pull away from the frame. Moisture can also cause wooden lath to decay or expand and contract. This movement often leads to cracks in the surrounding plaster. Sometimes, the keys themselves fail if they were not formed properly. If the laths are set too close together, the plaster cannot travel through to form a strong hook. This failure can cause walls to sag or even collapse.

Despite these risks, lath and plaster systems offer many benefits. They provide excellent soundproofing and help with heat insulation. Metal lath and plaster walls are especially impressive regarding safety. They can be twice as resistant to fire as standard drywall. A specific assembly can even achieve a two-hour fire rating. Additionally, two inches of plaster and lath can match the decibel rating of drywall for sound control. This makes lath a vital component in creating durable, safe, and comfortable living spaces.