How to find studs, and why a magnet beats the electronic finder

First work out what the wall is

Knock along it. Drywall over framing gives a hollow note between the studs and a duller, tighter one over a stud. Plaster over wood lath sounds solid nearly everywhere, which is why it defeats most stud finders and most people.

In Rome the age of the street tells you which you have. The older districts run plaster and lath. Anything built or renovated from roughly the sixties on is drywall. A house can be both, because a room added later is framed and boarded to whatever was normal that decade.

• Hollow between, dull on the line: drywall on wood framing, the easy case.
• Solid everywhere and hard to dent: plaster over lath. Electronic finders read the lath, not the stud.
• Cold and completely solid: masonry. There are no studs to find and it takes a different anchor entirely.

Why a magnet beats the electronic finder

An electronic finder does not find wood. It reads a change in density behind the surface and infers. Insulation, a pipe, a duct, a wiring run, a seam of joint compound and a second layer of board all change density, so all of them can read as a stud.

A magnet does something narrower and more reliable. It finds the steel fastener holding the board to the framing, and a fastener is only in one place: in a stud. It will not report a stud that is not there, because little else in a wall holds steel at that spacing. One exception is worth knowing: a steel nail plate, which is fitted precisely where a pipe or a wire passes through a stud to stop somebody driving into it. If the magnet stops somewhere that breaks the spacing, treat that spot as a service run and not as a fixing point.

A strong rare earth magnet is better than a magnetic stud finder and costs less. Let it hang on a string and drag it slowly across the wall at waist height. It stops on its own when it finds a screw.

Spacing tells you where the next one is

Framing is laid out on a repeating measurement, sixteen inches between centers in most houses and twenty four in some. Once you have one confirmed stud, the rest of the wall is arithmetic rather than searching.

Measure from the center of the one you found and mark at sixteen inches. Check it with the magnet. If it lands, the wall is on sixteens and you can mark the whole run. If it does not, try twenty four before assuming the layout is irregular.

Two found studs are worth much more than one, because two give you the actual spacing of this wall instead of the spacing it is supposed to have.

Where the rule breaks

Layout is regular in the middle of a wall and irregular everywhere else. Corners get doubled or tripled studs. Doors and windows are framed with a stud each side plus a header above, so the spacing restarts after an opening rather than carrying through it.

Old houses drift. A hundred year old wall framed by hand is close to sixteens rather than on them, and the drift accumulates: by the far end of a long wall the error can be most of a stud width.

Exterior walls are the awkward ones. Insulation, sheathing and sometimes a service cavity sit behind the board, and that is exactly the mix that makes an electronic finder guess.

Prove it before you commit

A stud is an inch and a half wide, so find both edges rather than one hit and mark the middle. A bracket screwed into the last quarter inch of a stud holds until the day it is loaded properly.

The honest final check is a small hole. A finish nail or a fine drill bit pushed through where the bracket will cover it either hits solid wood in half an inch or goes through into air. That is worth more than any reading, and the hole disappears under the plate.

This part is a magnet and ten minutes

Finding studs is homeowner work and the tools cost very little. A rare earth magnet, a tape measure and a pencil cover the whole job.

Where it stops being homeowner work is the load. A large television on a full motion arm puts a lot of leverage on its fixings, and the arm multiplies it every time it swings out. If the studs do not land where the bracket needs them, the answer is a backing plate spanning two of them, not a longer screw into the nearest thing that felt solid.

Plaster and lath deserves a mention on its own. Lath will hold a picture and nothing heavier, it cracks in a line when it is overloaded, and the repair afterward is plaster work rather than a dab of filler.

When to stop and call

Call when the bracket and the studs do not line up, when the wall is plaster or masonry, or when what is going up is heavy enough that being wrong means a repair rather than a rethink.

Also call if the magnet finds fasteners at spacings that make no sense, four inches apart, or in short clusters. That is usually a patch, a former opening or a second layer of board over the first, and none of those are what you want to hang a mount from.

Questions that come with this one