How to Find a Stud in a Plaster Wall
Older homes with lath-and-plaster walls are the toughest case for finding a stud. The plaster is thick, uneven, and can hide the framing from a typical finder. Let's be honest about why it's hard — and walk through the methods that actually work, including how your iPhone can pick up the metal nails that hold the lath to each stud.
What lath-and-plaster walls are
Before drywall became standard in the 1950s, interior walls were built from lath and plaster. Thin horizontal strips of wood lath were nailed across the studs, and wet plaster was troweled over them, oozing between the strips to lock in place. In later homes you may find rock lath (gypsum board lath) or metal lath (an expanded steel mesh) screwed or stapled to the studs instead of wood. In every version, the lath is fastened to the vertical wood studs with metal nails, screws, or staples — and that metal is the key to finding the framing.
Why plaster is tricky — the honest truth
It's worth being upfront: standard electronic stud finders and phone apps often struggle on plaster. Here's why. Plaster is much thicker and denser than drywall — sometimes an inch or more — so the studs sit far behind the surface. The plaster thickness is also uneven, which confuses density-based electronic finders that hunt for a change in wall stiffness; they frequently read false positives across the whole wall. If the wall has metal lath, that continuous steel mesh masks everything behind it and defeats most detectors entirely.
So don't expect the effortless single beep you'd get on drywall. The good news is that a magnetometer works differently: instead of sensing density, it senses metal. On wood-lath plaster, that means it can often pick out the line of metal lath nails driven into a stud. On rock lath or metal lath, it can find the screws or staples along the framing. That is a far more reliable approach on plaster than density sensing.
Methods that work on plaster
Find the lath nails. A magnetic detector — including the Stud Finder app on your iPhone — reacts to the metal fasteners holding the lath to each stud. Where you get a run of evenly spaced magnetic peaks stacked vertically, you've almost certainly found a stud.
Tap the wall. Knock across the plaster with a knuckle. Over a stud the sound is a dull, solid thud; between studs it rings slightly more hollow. It's imprecise on thick plaster, but it's a useful sanity check to pair with a scan.
Use outlets and trim. Electrical outlets and switch boxes are screwed to the side of a stud, so one edge of the box gives you a known stud location. Baseboards and window casings are also nailed into framing — clues you can build from.
Trust the 16-inch spacing. Even in old homes, studs are usually spaced roughly 16 inches on center (some are 24, and older houses can be irregular). Once you've confirmed one stud, measure over to predict the next, then verify it — never assume.
Find the stud in plaster, step by step
Start from a known reference
Locate the nearest outlet or switch. One side of its box is fixed to a stud, giving you a reliable starting point to scan out from rather than guessing across a blank wall.
Scan slowly for metal fasteners
Slide your detector horizontally and watch for a spike. On plaster you're looking for the metal lath nails or screws that hold the lath to the stud — not a change in wall density. Move slowly; the signals are subtler than on drywall.
Look for a vertical run of peaks
A single peak could be a random nail. Scan a few inches higher and lower — a real stud shows a column of evenly spaced fastener signals running vertically. That repeating pattern is your confirmation.
Measure 16 inches and verify the next stud
From a confirmed stud, measure roughly 16 inches across and scan again. In older homes spacing can drift, so treat the measurement as a hint and always re-scan to confirm the real location.
Confirm before a heavy load
Tap the spot for a solid thud, then — for anything heavy — drill a small pilot hole. If the bit hits firm wood after the plaster and lath, you're on a stud. Anchors alone won't reliably hold weight in plaster.
Scan lath nails with your iPhone
Because plaster defeats density sensing, a magnetic approach is your best bet — and that's exactly what your phone does. The Stud Finder – Metal Detector app uses your iPhone's built-in magnetometer to detect the metal nails, screws, and staples fastening the lath to each stud. Here's how to work it on a plaster wall:
- Remove your phone case so the sensor sits as close as possible to the thick plaster surface.
- Hold the phone flat against the wall near an outlet and tap start to establish a baseline reading.
- Slide slowly and horizontally. Watch the live meter climb toward orange — a peak with its alert marks a lath fastener in a stud.
- Scan up and down a few inches to confirm a vertical line of peaks, then mark the stud and measure over for the next.
Scan your plaster wall free
Pick up the lath nails with the magnetometer already in your iPhone.
Mistakes to avoid
- Expecting drywall-easy results. Plaster is thicker and noisier — scan slowly, confirm vertically, and never trust a single beep.
- Mistaking a lone nail for a stud. Random plaster keys and trim nails exist. Only a repeating vertical run of peaks reliably marks a stud.
- Ignoring metal lath. If the whole wall reads as metal, no magnetic finder can isolate studs — switch to a small test hole instead.
- Relying on anchors for weight. Plaster crumbles. For a heavy mirror or picture, land your fastener in real wood, not the plaster alone.