These two words get thrown around a lot with older buildings. And while someone may recognize a photo of a wall built this way, there is a whole world of history, maintenance and repair that are often glossed over.
Early human built structures relied upon the materials of the structure for the finish. Think log and stone cabins; What you see is what you get–on both sides. As our massive brains figured out more materially and labor economical ways to construct spaces, timber/lumber and bricks being the most popular, we lost the benefit of having a pre-finished interior surface. Even with a lot of early structures, there was still a desire for a different surface treatment on the interior. This is where plaster came in.
A mixture of lime with sand and/or cement creates the variety of plasters out there. When used on the exterior of the structure, it then becomes stucco. Depending on the desired finish and location of use, the ratios of these materials may change. Fibers, often horsehair, would be added regularly to help improve strength of these coats.
With solid stone or brick construction, it is easy to apply plaster directly to the construction surface. Once you move to a frame based construction, your home is now filled with large gaps that must be overcome. Solid wood, as you could imagine, would be very expensive. So a product of economic and functional forces was: lath. Thin strips of wood would be nailed to the lumber to create a semi-solid surface that plaster could be applied to. Voila!
Once the lath was nailed in place, plaster would be mixed on the spot, and applied directly to the lath. This is where varying the mixture of plaster becomes important. This first layer would have more sand and other binders in it to help it stick to the lath. The spacing of the lath was also key, allowing some of the plaster to flow between the slats. Once hardened, these extruded pieces are referred to as keys. This is where the strength (or weakness, as we will see) of a plaster wall originates.
Once this base coat is complete a finish coat, or two, would be applied. Frequently vertical guides would be installed to ensure a flat and square surface. Since most houses with plaster and lath were built before standardized dimensional lumber, there would be variations from stud to stud. This application method made it easier to absorb these differences into the thickness of the plaster being applied.
Beyond making up for wall thickness variations, plaster and lath application afforded many other opportunities. Curved and decorative surfaces were significantly easier to apply with a wet medium. Do a quick image search for plaster crown molding or ceiling medallions to get an idea.
P+L is also significantly harder and denser than the drywall that replaced it. You end up with significantly less noise traveling room to room, and it is much hard to scratch or dent from an errant piece of furniture
Maintenance + Repair
There are a thousand and one tutorials on repairing cracks/holes/etc with P+L out there. I won’t rehash them, but I will emphasize understanding of the the original installation as crucial to proper ongoing repairs.
Almost all plaster and lath issues stem from the plaster surface becoming detached from the keys through the lath. This could be from physical trauma of a past remodel or alteration, or simply from the movement of the house over time. Water damage can also frequently cause separation of these components. With any issue where keys are broken or missing, the remedy is going to involve re-adhering the plaster to the lath. No matter how much “spackle” you put over that crack or loose section, until you re-adhere, it is going to return.
An easy way to test a plaster wall it to simply apply pressure to it, slowly, and see if the plaster flexes. This will often be accompanied with an acoustic component as the plaster interfaces with the lath where the keys used to be. Any project I have started a walked through on always incorporates this in every room. You don’t need to be scared of plaster and lath, you just have to know what you’re dealing with.