Lath and plaster walls grace many traditional homes. Thick, substantial, and great at soundproofing, walls constructed out of lath and plaster are rarely built anew anymore. They tend only to be repaired, not built from scratch. Drywall has supplanted plaster and lath as the wall covering of choice. But if your home does have this older interior wall-building system, you may be curious about how it is constructed and how it compares to drywall.
What Is a Lath and Plaster Wall?
Lath and plaster refer to an interior wall construction technique that typically predates the 1940s. Four-foot-long strips of wood lath, typically 1-inch wide, are nailed directly to the open wall studs. The lath is then embedded with three layers of wet plaster. The plaster dries to form a hard, smooth surface suitable for finishing, first with primer, then with interior paint or wallpaper. Plaster and lath wall systems are rarely used now, except to repair existing walls or to refurbish historic buildings.
In the mid-century modern era after World War II, drywall, also called plasterboard or wallboard, stormed onto the scene and has remained there ever since.
Essentially, drywall acts the same way as lath and plaster walls: a mineral-based substance that creates privacy, deadens sound, and insulates. The chief difference is that the plasterwork, or the wet work, has already been done in a factory, not on site. Thus, the term drywall. Also, since drywall is stiff, lath backing is not necessary. Drywall can be nailed directly onto the studs.
Lath and Plaster vs. Drywall
Lath and plaster walls are usually thicker than most drywall sheets. Fire-rated, or Type-X, drywall is 5/8-inch thick. Plaster is often thicker than this. When lath is figured into the thickness, then lath and plaster walls are considered to be thicker than drywall. More importantly, drywall has consistent thickness, while lath and plaster systems can have inconsistent thicknesses that result in gentle ridges and dips.
Soundproofing and Insulation
Lath and plaster walls provide a room with better soundproofing, as opposed to drywall walls of an ordinary thickness (1/2-inch). Soundproof drywall, though, rivals lath and plaster walls in terms of soundproofing. Lath and plaster walls have a slightly better insulating R-value than do drywall wall systems.
Though lath and plaster walls' materials are inexpensive, they are much more expensive to build because they require skilled labor. Drywall is inexpensive to hang and finish.
Homeowners can repair lath and plaster walls by themselves using drywall materials and tools. A few companies that specialize in finishing drywall may be able to repair plaster walls, as well. Large metro areas that have a large stock of older homes tend to have tradespeople who specialize in plaster application and repairs. Drywall is easy to repair. Damaged drywall can be sectioned out and replaced by new pieces of drywall.
How Lath and Plaster Walls Are Built
Building lath and plaster walls requires several days due to the length of time it takes for thick plaster to cure. Though lath-building can nominally be thought of as carpentry, most plasterers are adept at nailing up lath. So only one team of workers is needed for the project.
Lath Is Nailed to the Studs
A substrate in the form of a grid of lath is nailed perpendicular to the open house studs roughly a finger-width apart from each other. Lath is rough, unfinished wood about 1-inch wide and about 4 feet long or even longer. Longer lath strips allow the lath to go up faster. But since lath is ripped from lower-quality wood, long sections of acceptable wood may not be available. Sometimes, in lieu of wooden lath, this base surface is a metal grid.
Plaster Is Added to the Lath
A thick layer of wet plaster is hand-troweled onto the lath. Typically, this layer is actually composed of three thinner layers: scratch coat, brown coat, and white coat. The scratch coat forms the plaster keys (next step) and provides basic coverage. The brown coat is for further flattening and plumbing (making vertical) the walls. The white coat provides a smooth surface for painting or wallpapering. Sometimes, the white coat is textured.
What Are Plaster Keys?
Plaster keys are simply the plaster that has oozed through the lath in irregular shapes. They help to hold the plaster in place on the lath, as well as act as sound-absorbing elements in the wall.
Keys Develop Behind the Lath
Globs of plaster called keys are pushed out of the backside of the lath. These wet portions dry and form myriad secure grips for the rest of the plaster. The keys make demolition of plaster down the road much harder because they grip to the lath.
Wall Is Primed and Painted
Plaster is highly porous and must be primed prior to painting.