In this article
Plaster walls show up in the oldest houses we work in: 1920s homes in Coral Gables and Miami Shores, Victoria Park in Fort Lauderdale, El Cid in West Palm Beach, and pre-war bungalows in Los Angeles. They are harder than drywall, often an inch or more thick, and they make electronic stud finders useless. Here is how we find the framing anyway.
1. The magnet method (our first choice)
Wood lath strips were nailed to every stud. Those nails are still there. Slide a strong rare-earth magnet slowly across the wall at hip height; when it grabs, mark the spot. Repeat higher and lower — the nails line up vertically along the stud. On most houses you will find a stud every 16 inches, sometimes 24.
2. The test hole
Once the magnet suggests a stud, drill a 1/16-inch hole in a spot the mount plate will cover. You will feel plaster (gritty), lath (softer), then either a void or solid wood. Solid wood a little past an inch deep means you found the stud.
3. The probe wire
If the test hole hits a void, bend a stiff wire into an L, push it through the hole and rotate it until it touches wood. The length of wire to the right or left tells you where the stud is.
4. Outlets and switches
Electrical boxes in old houses are almost always nailed to the side of a stud. Turn off the power, remove the cover plate and look into the gap beside the box — you can often see which side the stud is on.
5. The knock test
Knock with a knuckle along the wall. Over a stud the sound is shorter and duller. On plaster this is less reliable than on drywall, but it helps confirm what the magnet says.
6. Baseboard and trim nails
Baseboards and crown moldings were nailed into studs. The small filled nail holes in the trim give you the stud spacing at floor and ceiling level.
Plaster cracks if the drill walks. Put painter's tape over the spot, use a sharp bit and start slow. Switch to a masonry bit if you hit old gypsum plaster that dulls wood bits.
Fastening into the stud
Plaster and lath add roughly an inch before the wood. Use 5/16-inch lag bolts 3-1/2 to 4 inches long, with a 3/16-inch pilot hole, so at least two inches of thread bite into the stud. The plate must span two studs. Our mount picks for plaster list plates wide enough to reach.
When the house is masonry, not wood
In South Florida, "plaster wall" often means plaster applied directly to concrete block or clay tile — no studs at all. If the test hole brings up gray or red dust instead of wood, switch to masonry anchors: see mounting on concrete.
Rather have a pro do it?
Exact price in about a minute — TV size, wall type and cables included. Same-day in most areas, 3-year warranty on every install.
Old plaster is one of the walls we handle every week. Installation starts at $119, and plaster costs the same as drywall — the calculator shows the exact total.
Gear mentioned in this article
Franklin Sensors ProSensor M210 13-Sensor Stud Finder, Live Wire DetectionStud finder: locates the framing behind the drywall.Check price →
Swanson 9” Die Cast Aluminum Torpedo Level with Rare Earth Magnetics TL002MLevel: gets the bracket straight on the first try.Check price →
Prime-Line 9056277 Hex Lag ScrewsLag bolts for wood studs.Check price →
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