If your house went up before about 1950, your walls are probably plaster over wood lath — thin horizontal strips nailed to the studs, then coated with plaster that squeezes between them. It's a tough, quiet wall. It's also the wall that makes a $30 stud finder cry. The finder isn't broken, exactly. It's just reading a wall it was never designed for.
The good news: a plaster wall was built in a pattern, and the pattern is still in there. The lath had to be nailed to something, and that something was the studs. Find the nails and you've found the framing. Here's how, in order of reliability.
A basic electronic finder works on drywall because drywall is uniform — same thickness, same density — so the bump in readings over a stud stands out. Plaster breaks every part of that. The coating varies from about 3/8 inch to a full inch thick. Behind it sit wood strips with gaps, plaster keys bulging through those gaps, hundreds of nails, and often wire mesh in the corners or metal lath patches from old repairs. The sensor sees constant variation, so it beeps everywhere, nowhere, or at a pipe strap. Deep-scan modes don't help. The problem isn't depth. It's noise.
Lath strips run horizontally, and each strip was nailed at every stud it crossed. That means the nails over one stud form a vertical column, repeated every couple of inches from floor to ceiling. A magnet dragged over that column tugs again and again. Between studs, it barely pulls at all.
Use a magnet with real pull. A neodymium magnet the size of a coin can feel a nail through 3/4 inch of plaster. Hold it flat against the wall, or tie it to a short string and let it swing and catch as you drag it.
Sweep slowly at mounting height. Move the magnet in slow horizontal passes where you plan to hang. You'll feel a rhythmic drag-tug, drag-tug over a stud's nail column — and a smooth glide over the open bay between studs.
Mark the pattern, not a single tug. Put a small piece of painter's tape at each strong tug. One tug means nothing — it could be a stray nail or a repair screw. A vertical line of tugs at regular intervals is a stud announcing itself.
Check the spacing before you trust it. Measure over to where the next stud should be — usually 16 inches — and sweep there. If the magnet finds a matching column, your marks are framing. If it doesn't, slide left or right; old walls wander.
Baseboards were nailed into the studs, and a century of paint rarely hides the evidence completely. Run your eyes and fingertips along the baseboard for filled nail holes — slight dimples or bumps, often in pairs near the top edge of the board. Then run the magnet along that same top edge: nails will cluster at the studs. Door and window casing works the same way. Once you have a stud at floor level, carry the line straight up the wall with a level or a long straightedge.
One caution: the bottom of the baseboard may also be nailed to the horizontal plate that runs along the floor, so hits right at floor level, evenly spaced, don't count. Trust the clusters near the top edge.
Electrical boxes mount to the side of a stud, so every outlet and switch is sitting an inch or so from framing. The magnet tells you which side — sweep beside the box and the lath nails will show up on the stud side. Corners are another anchor point, since walls frame out from them. From any confirmed stud, measure your 16-inch increments along the wall, then test each predicted spot with the magnet. Treat the measurement as a hypothesis and the magnet as the test, because additions and repairs in old houses break the rhythm without telling you.
Plaster forgives a pin hole and hides it with a dab of spackle, so verify before you drill for anchors. Pick a spot that will be covered — behind the mount itself, down low behind furniture, or on the closet side of the same wall, where the stud line is identical. Drill slowly with the small bit. You'll feel gritty resistance through the plaster and lath, then one of two things: the bit bites into wood and the dust turns to pale shavings (that's a stud), or it drops suddenly into a hollow, dusty cavity (that's a miss). On a miss, move an inch and test again.
| Method | What it actually finds | Reliability on plaster | Watch out for |
|---|---|---|---|
| Magnet on lath nails | The nail column over each stud | High — physical proof | Metal lath, which pulls everywhere |
| Baseboard nails | Stud positions at floor level | Good, then extend the line up | Bottom-plate nails at floor level |
| 16-inch layout | The framing pattern | A starting guess only | Old walls that drift off pattern |
| Test hole | Wood or hollow cavity, directly | Definitive | Keep it hidden and shallow |
Trusting the finder anyway. Scanning three times and averaging the beeps doesn't fix a sensor that's reading noise. On plaster, a consistent lie is still a lie.
Bringing a fridge magnet. Decorative magnets can't pull through an inch of plaster and lath. Weak magnet, no tugs, wrong conclusion — the method gets blamed for the tool.
Assuming perfect 16-inch spacing. Plenty of pre-war walls run 15, 17, or even 24 inches in spots. Measure from a confirmed stud on that wall, not from a rule in a book.
Drilling the anchor hole first. The test hole takes 20 seconds and hides behind the mount. The wrong anchor hole takes a patch, a sanding, and a paint touch-up. Sequence matters.
On plaster and lath, forget density scans and work with what's physically in the wall: nails, pattern, and one tiny test hole. Magnet first, tape measure second, drill last. If you're weighing tools for drywall elsewhere in the house, our comparison of stud finder vs magnet covers where electronics do win, and the no-tool methods in finding a stud without a stud finder apply once drywall enters the picture. With your stud verified, mount with confidence — the same verification habit is what keeps a heavy TV on the wall in our guide to mounting a TV so it doesn't fall.
Because it reads density changes, and plaster over lath is nothing but density changes — variable plaster thickness, wood strips, gaps, keys, and hundreds of nails. On uniform drywall the stud stands out. On plaster, everything stands out, so nothing does.
Yes, if it's a strong neodymium magnet. You're not detecting the wood — you're detecting the steel nails that fasten the lath to the stud. They sit in a vertical column over each stud, and the magnet tugs at each one as you pass over it.
Sixteen inches on center is the most common layout, but pre-1950 framing is less disciplined — 15, 17, and 24-inch spacing all turn up, sometimes in the same wall. Always measure from a stud you've confirmed on that wall rather than trusting the standard.
You probably have metal lath instead of wood lath — common in later plaster work and in many bathrooms. The magnet can't isolate nails in a sheet of steel mesh, so fall back on the layout: measure from an outlet or corner and settle it with a test hole.
Somewhere the finished mount, furniture, or baseboard will cover it — or on the closet side of the same wall, where the stud lands in the same place. Keep it small (1/16 or 3/32 inch), drill only through the plaster, and fill it with a dab of spackle afterward.