Stud detector: setting it up and changing the bit or blade

Learn to scan a wall for hidden studs and cables safely, then change the drill bit ready to fix into it.

SD-01Know what the detector actually findsA stud detector (also called a wall scanner) is a handheld device that senses changes behind plasterboard as you slide it along the wall - things like timber studs (the vertical wooden or metal uprights inside a wall), joists (the horizontal beams under floors), pipes, and sometimes live cables. It works by picking up small changes in density or, on better models, magnetic and electrical signals. The beginner mistake is treating it like an X-ray machine that shows everything with certainty - it doesn't.

SD-02Calibrate it on a clear patch firstMost detectors need zeroing before use. Hold it flat against the wall in a spot you're fairly sure is clear - away from corners, sockets, and light switches - then press and hold the calibration button until the light or beep settles to a steady 'clear' signal. If you accidentally calibrate over a stud, the detector will read everything else as false, so pick an open stretch of wall if you can.

SD-03Scan slowly in one directionKeep the detector flat against the wall and slide it slowly and steadily sideways, in one direction only. Don't scan back and forth - it confuses the reading. Watch the lights or listen for the beep to change, and the moment it does, stop and mark that edge lightly with a pencil.

SD-04Scan again to find the centreStart a few centimetres past your mark and scan back the other way to find the second edge of the stud. Mark that edge too, then find the middle point between your two marks - that's the true centre, not the first beep you heard. The common mistake is drilling on the first signal, which often catches only the edge of the stud rather than its middle.

SD-05Treat the reading as a warning, not a guaranteeA detector reduces risk, but it does not remove it. It can miss cables that run deep, sit inside metal conduit, hide behind metal-backed plasterboard, or simply weren't installed in the 'safe zones' that modern wiring rules expect - older or amateur wiring often ignores those zones completely. Always assume there could be a cable or pipe there and prove otherwise, rather than assuming a clear reading means it's safe. If there's any doubt at all, isolate the circuit at the consumer unit (the fuse box) before drilling, and turn the water off at the stopcock before cutting into anything that could carry water.

SD-06Open the chuck and swap the bitThe chuck is the part at the front of the drill that grips the bit. On most modern drills it's a keyless chuck - twist the front collar anti-clockwise by hand until it opens wide enough to pull the old bit out. Push the new bit in straight and as far as it will go, then twist the collar clockwise by hand until it's tight, giving the trigger a very brief tap to help it seat firmly.

1 each · Masonry or wood drill bit (size to suit job)

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SD-07Check the bit is fitted trueSpin the bit by hand first - it should turn smoothly with no visible wobble at the tip. Then, without touching the wall, pull the trigger for a second and watch the spinning tip: if it looks like a single sharp point rather than a blur or circle, it's fitted straight. A wobbling tip means the chuck hasn't gripped it evenly, so open it and reseat the bit before you drill anything.

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