Which wall detector do I need, and what will it miss?

Detectors are sold as one product and are really several, each using a different physical principle with different failure modes. Knowing which mode is doing the work at any moment is what turns a device that people distrust into one that is genuinely useful — and, more importantly, tells you when it is telling you nothing.

Safety

Step by step

1. Buy a multi-detector, not a stud finder.The extra cost over a bare stud finder is small and the live-cable channel is the one that matters for safety. A stud finder alone tells you where to fix and nothing about what you might hit.
2. Calibrate it on a clear area, every time.Most electronic units zero themselves against whatever they are switched on over. Starting over a stud, a pipe or a cable makes every subsequent reading wrong across the whole wall, and it looks completely normal.
3. Sweep slowly, in both directions, in both axes.Much slower than feels natural. Sweep left to right and right to left to find both edges of anything, and sweep vertically as well as horizontally — a horizontal sweep can run parallel to a cable and never cross it.
4. Switch modes deliberately and re-sweep.A scan in stud mode says nothing about metal. A scan in metal mode says nothing about whether a cable is live. Do each mode over the whole area you are interested in, not one mode over the whole wall.
5. Know what it cannot see.Plastic push-fit pipe — invisible to everything. A dead cable — invisible to live-cable mode, though its copper may show on metal. Anything beyond the unit's depth, which is often less than the wall thickness. And anything at all behind foil-backed insulated plasterboard or metal lath, because the surface itself reads as continuous metal.
6. Reason about the building as well.What is on the other side of this wall? A kitchen or bathroom means pipes. A socket below means a cable rising. A boiler nearby means both. Reasoning finds things a detector cannot, and a detector finds things reasoning cannot.
7. Confirm anything important physically.A strong magnet on a string locates plasterboard screws reliably. A fine bradawl or a 2mm bit pushed in gently through a spot you will cover confirms a stud. Both are slower than a detector and both are more certain.
8. Drill shallow and increase depth in stages.Set a depth stop, go in a few millimetres at a time, and stop the moment anything feels different. This is the last line of defence and it works — a drill entering copper or a cable sheath feels different long before it goes through.

Tips

• Detectors read differently on a wet or freshly plastered wall, on textured finishes, on wallpaper with a metallic component, and through tiles. If readings are erratic, the surface is usually why.
• Mark everything you find with masking tape rather than pencil. It is visible from across the room, it comes off cleanly, and it survives the plaster dust.
• For a wall where getting it wrong is expensive — a kitchen, a bathroom, near a consumer unit — isolating the circuit at the consumer unit before drilling removes the electrical risk entirely, at the cost of working by torchlight.
• The deep-scan mode on most units trades resolution for depth. Use it to know that something is there, then use normal mode to find exactly where.

Common mistakes

• Treating a clear scan as permission to drill anywhere — The four blind spots — plastic pipe, dead cable, depth and foil-backed board — cover a large share of real houses. A clear scan means the detector found nothing, not that there is nothing.
• Sweeping in one direction only — A single pass gives you an edge, not a centre, and a sweep parallel to a cable may never cross its field at all.
• Skipping calibration — It is the single commonest reason a detector is dismissed as useless. Calibrated over a stud, it treats the stud as normal wall and reports studs everywhere else.
• Trusting it near a consumer unit or a boiler — Both are places where several cables converge in unpredictable directions and where the consequences of a mistake are worst. Neither is a place to rely on any detector.

Alternatives

• Magnetic stud finder: A rare-earth magnet, sometimes on a pivot, that locates the screws or nails holding plasterboard to the studs. No batteries, no calibration, nothing to go wrong, and very reliable for what it does. It finds fixings, not timber, and it finds nothing electrical.
• Electronic stud detector: Senses the change in density behind the surface. Finds timber and metal studs and their edges, and is the mode most affected by wall thickness, wallpaper, textured finishes and damp. Needs calibrating on a clear area every time.
• Metal detection: Finds ferrous and non-ferrous metal — copper pipe, steel studs, conduit, screws, reinforcing mesh, lintels. It does not know whether the metal is a pipe or a nail, and it is saturated by foil-backed board or metal lath.
• Live cable (AC) detection: Senses the alternating electromagnetic field around an energised conductor. Only finds cables that are switched on and carrying, or at least energised. Range is limited and it is affected by how the cable is run and what is around it.
• Multi-detector: All of the above in one unit, usually with a deep-scan mode. The right thing for most people to own, provided you understand that switching mode changes what it can physically see.
• Non-contact voltage tester: Not a wall detector — a pen-shaped tester that indicates the presence of mains voltage at a conductor or an accessory. Useful alongside a detector for a different question, and it has its own significant limitations.

Questions people ask