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Knock on a wall in a Chicago two-flat and it answers with a dense, dead thud. Knock in a 1990s condo and it drums. That difference is a whole chapter of building history: every wall here was plaster for a century, then almost overnight everything became drywall. This page explains how each one is built, how to tell which you are looking at, why the switch happened, and what it all means when you own the walls.
The calendar mostly decides it. Built before World War II, the walls are almost certainly plaster. Built after about 1960, drywall. The 1940s and 50s are the in-between years, when builders used rock lath, a hybrid you will meet below. Gut-renovated vintage buildings are the exception that keeps you guessing: a greystone shell with brand-new drywall inside is common, and so is a vintage unit where one remodeled room drums while the rest of the house thuds.
Rap a wall with a knuckle. Plaster is dense and about an inch thick with its lath, so it answers with a solid, dead knock and feels cold and hard. Drywall is a half-inch panel over hollow bays, so it sounds hollow and drummy between studs. Ten seconds at a showing tells you what a listing never mentions.
Tap any part of either drawing; the panel explains it.
Plaster is not a board, it is a masonry skin built in place, and watching the order of operations explains everything about how it behaves. First came the lath: thin wood strips nailed across the studs with deliberate gaps between them, thousands of them in an ordinary flat. Then a plasterer troweled on the scratch coat, a coarse wet mix pressed hard so it squeezed through the gaps and mushroomed behind the lath. Those hardened mushrooms are called keys, and they are literally what holds the wall up: the plaster is hanging on by thousands of little hooks.
A second coat, the brown coat, flattened the surface true. A final finish coat, a thin skin of lime putty polished with a trowel, gave the glassy surface you can still feel on an original wall. Three coats, days of curing between them, weeks start to finish, all of it skilled handwork. Later buildings swapped wood strips for metal mesh, and by the 1940s for rock lath, small gypsum panels used as the plaster base, which is why some mid-century walls are plaster-smooth but only three quarters of an inch thick.
Drywall inverted the whole idea: instead of building the surface wet on site, the factory builds it and the crew just hangs it. Gypsum panels, four feet wide, get screwed straight to the studs in an afternoon. The seams and screw dimples then get taped and buried under joint compound, three passes with drying and a wider knife each time, then sanded smooth and primed. Where a plaster crew needed weeks and a real trade, a drywall crew needs days, and the finishing, the taping and mudding, is where all the remaining skill lives. A cheap job and a great job use the same boards; the mud tells them apart.
One naming note so the aisles make sense: Sheetrock is a brand of drywall the way Kleenex is a brand of tissue, and the blue-paper moisture-resistant boards in bathrooms and the cement boards under tile are cousins in the same family.
Drywall existed for decades before anyone respected it; it was the cheap stuff. Two things flipped the market. The war pulled the skilled trades into service while factories and housing still had to go up fast, and drywall needs far fewer hands and no curing time. Then the postwar boom demanded houses by the tens of thousands, and a material that shaved weeks off every single one was never going to lose. By the 1960s plaster had become the specialty and drywall the default, and the price of a real plasterer has been climbing ever since, which matters the day you need one.
Plaster earned its century. The wall is heavier and harder, so it deadens sound in a way a hollow drywall partition cannot, one real reason vintage flats feel quiet. The lime-and-gypsum skin is more fire-resistant than paper-faced board. And because it was applied wet by hand, it could do shapes: the curved archways, coved ceilings, rounded bullnose corners, and ceiling medallions in Chicago vintage stock are all just plaster doing what a flat panel never will. Even the picture rail near the ceiling in old rooms is part of the system: it exists so you can hang art from hooks without ever nailing the wall.
Its personality quirk is cracking. Plaster is rigid on a building that moves with seasons, so fine hairlines around openings are ordinary aging, not alarm. The crack patterns that mean something structural, wide, stair-stepped, or growing, are the same ones our foundations guide teaches you to read.
The repair ladder runs in order. Hairline cracks get flexible filler and paint. Cracks that reopen every season get taped and skimmed like a drywall seam. Where plaster has gone loose from its lath, plaster washers, wide flat screws, pull it snug again, and conservators use adhesive injection for the same job invisibly. Whole tired walls get a skim coat, a thin new finish over the old, or get overlaid with thin drywall screwed through to the studs, which keeps the mass and buries the cracks. Full demolition is the last resort, not the default: it is brutally dusty, it throws away the soundproofing, and in pre-1978 buildings those paint layers are a lead job, which our hazards guide covers.
The one plaster condition to take seriously on the spot: a ceiling that sags or bellies. That means the keys, those hooks behind the lath, have sheared, and the plaster is holding on by habit. A bulging plaster ceiling can come down at once and it is heavy. Do not push on it to check. Note it, photograph it, and price the repair before attorney review ends.
Plaster changes the arithmetic of every later project. Electricians fishing new wires fight the lath and the rock-hard keys, so wiring upgrades in vintage buildings cost more than the same work in drywall, a thing our electrical guide touches from its side. Insulation gets blown in behind plaster through small patched holes rather than by opening walls. Matching a curved plaster detail is artisan work, so renovations tend to preserve the curves and spend the money where it shows. None of this is a reason to fear plaster; it is a reason to price vintage projects with people who have done them before, and we keep a bench of exactly those.
Money,: a hairline patch is a handyman line item, a room skim coat is a modest project, and a full skim or overlay of a vintage unit is real money worth three quotes, with true three-coat restoration above that and worth it in landmark-grade rooms. On the drywall side, hanging is cheap and finishing is the art, so the difference between bids is usually the mud, not the board.
Keep reading the building: buying something older for the whole-house version, windows for the openings in these walls, and the spotter’s guide to put wall reading on your walk-through list.