What is lath and plaster?

What is lath and plaster made of?

Lath and plaster has two parts: lath, the strips or sheets fixed to the framing, and plaster, a mix of a binding material (lime, later gypsum), sand and, in early lime plaster, animal hair, put on in coats. The National Park Service describes the plaster this way: “Lime plaster was made from four ingredients: lime, aggregate, fiber, and water.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The lime came from ground-and-heated limestone or oyster shells; the aggregate from sand; and the fiber from cattle or hog hair.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Gypsum and lime plasters were used in combination for the base and finish coats during the early part of the 20th century; gypsum was eventually favored because it set more rapidly and, initially, had a harder finish.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

How does the National Park Service describe lath and plaster?

The National Park Service treats lath as the base that holds the plaster on the wall, and plaster as the hardened coating over it. On its Truman Home page it describes the oldest and commonest base, wood lath, this way: “Lath provided a means of holding the plaster in place.” Source: U.S. National Park Service, Harry S Truman National Historic Site, read 2026-10-30. “Wood lath is usually made up of narrow thin strips of wood with spaces in between.” Source: U.S. National Park Service, Harry S Truman National Historic Site, read 2026-10-30.

How does lath and plaster work?

The plasterer nails lath to the joists or studs, then presses wet plaster onto it so some squeezes through the gaps and slumps down on the back side. It hardens there into keys that hold the plaster on the wall, and the full job is built up in coats. The NPS brief explains: “Wooden lath was nailed at right angles directly to the structural members of the buildings (the joists and studs), or it was fastened to non-structural spaced strips known as furring strips.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The plasterer applies a slight pressure to push the wet plaster through the spaces.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The plaster slumps down on the inside of the wall, forming plaster "keys."” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “These keys hold the plaster in place.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The first two coats made up the coarse stuff; they were the scratch coat and the brown coat.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Compared to the 3/8-inch-thick layers of the scratch and brown coats, the finish coat was a mere 1/8-inch thick.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

What is metal lath and plaster, and how does it differ from wood lath?

Metal lath is a steel base with many more openings than wood strips, which began to be used in parts of the United States toward the end of the 19th century; a third base, rock lath, was a gypsum board with a paper face. The NPS brief gives the minimum plaster thickness each base needs, from 1/2 inch on rock lath to about 3/4 to 7/8 inch on wood lath: “Metal lath, patented in England in 1797, began to be used in parts of the United States toward the end of the 19th century.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The steel making up the metal lath contained many more spaces than wood lath had contained.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “These spaces increased the number of keys; metal lath was better able to hold plaster than wood lath had been.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “In use as early as 1900, rock lath was made up of compressed gypsum covered by a paper facing.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “The minimum total thickness for plaster on gypsum board (rock lath) is 1/2 inch.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “On metal lath the minimum thickness is 5/8 inch; and for wood lath it is about 3/4 to 7/8 inch.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

When was lath and plaster used?

Lath and plaster was the standard way to finish interior walls in American buildings until about the 1930s and 1940s. Two National Park Service sources say so, and the brief adds when the lath itself changed: “Plaster was the dominant interior wall finish until the 1940s due to its versatility and strength.” Source: U.S. National Park Service, Harry S Truman National Historic Site, read 2026-10-30. “This variety and the adaptability of the material to nearly any building size, shape, or configuration meant that plaster was the wall surface chosen for nearly all buildings until the 1930s or 40s (Fig. 1).” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Until the end of the 19th century, plasterers used lime plaster.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “By the late 1930s, rock lath was used almost exclusively in residential plastering.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

Is lath and plaster still used?

Plastering has not stopped. As of its 1989 publication, the National Park Service brief says plasterers work mostly in gypsum plaster, that a special gypsum is made for fibered base coats, and that lime putty is still used for finish coats: “In general, gypsum plaster is the material plasterers use today.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Today, if fiber is required in a base coat, a special gypsum is available which includes wood fibers.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Lime putty, mixed with about 35 percent gypsum (gauging plaster) to help it harden, is still used as the finish coat.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

How long does lath and plaster last?

Sound plaster on sound lath can last for generations; the National Park Service says dry plaster should last almost indefinitely. What ends it early is a failed bond with the lath, which the brief ties to moisture, poor quality plaster or overloading. It says: “When plaster dries, it is a relatively rigid material which should last almost indefinitely.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Keys may have deteriorated because of a localized moisture problem, poor quality plaster, or structural overloading; yet, the surrounding system may be intact.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

Why does lath and plaster crack?

Cracks come from the house moving or from the lath itself: stress cracks run diagonally from door and window corners, horizontal cracks are often caused by lath movement, and wood lath swells and shrinks with humidity. The NPS brief says: “Appearing as diagonal lines in a wall, stress cracks usually start at a door or window frame, but they can appear anywhere in the wall, with seemingly random starting points.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Horizontal cracks are often caused by lath movement.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Because it absorbs moisture from the air, wood lath expands and contracts as humidity rises and falls.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “Overloading and structural movement (especially when combined with rotting lath, rusted nails, or poor quality plaster) can cause plaster to detach from the lath.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “When the mechanical bond with the lath is broken, plaster becomes loose or bowed.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30.

Does lath and plaster contain asbestos?

Sometimes. The National Park Service says asbestos, used in the mid-twentieth century as an additive, may be present when plaster is removed, and the Minnesota Department of Health lists plaster among asbestos-containing materials found in homes. A sample, not a look, settles it. The sources say: “Asbestos, used in the mid-twentieth century as an insulating and fireproofing additive, may also be present and OSHA-recommended precautions should be taken.” Source: U.S. National Park Service, Technical Preservation Services, read 2026-10-30. “You can check for asbestos markings on the material or its packaging, or hire a Minnesota-certified asbestos inspector to sample the material or perform an asbestos inspection.” Source: Minnesota Department of Health, read 2026-10-30. “If the asbestos material is in good condition and if it is not being disturbed, then it will not release asbestos fibers.” Source: Minnesota Department of Health, read 2026-10-30.

Sources

• Preservation Brief 21: Repairing Historic Flat Plaster Walls and Ceilings — U.S. National Park Service, Technical Preservation Services; published 1989-10; read 2026-10-30.
• Plaster and Lath — U.S. National Park Service, Harry S Truman National Historic Site; published 2015-04-10; read 2026-10-30.
• Asbestos in Minnesota Homes — Minnesota Department of Health; read 2026-10-30.