How do I check what is behind a wall before cutting a hole in it?

A drill makes a hole you can stop making. A multi-tool, a padsaw or a hole cutter makes a hole you cannot un-make, and it does it fast, through several things at once, in a place you cannot see. The difference in consequence is why cutting an opening deserves a proper inspection sequence rather than a quick scan.

Safety

What you'll need

• Multi-detector
• Torch
• Small inspection camera or a dentist's mirror (optional)
• Padsaw or a small drill for the inspection hole
• Masking tape and pencil

Step by step

1. Establish what kind of wall it is first.Knock it. A hollow sound with a regular pattern of solid spots is a stud wall. A dead solid sound is masonry. A slightly springy, dusty solid is lath and plaster. Dot-and-dab plasterboard on masonry sounds hollow but has a shallow void and a solid wall behind, which changes everything about what could be in it.
2. Establish whether it is load-bearing before you plan anything large.This matters for openings, not for a socket box. If the wall runs at right angles to the floor joists, continues on the floor above, or sits over a beam or a wall below, treat it as structural until an engineer or a builder says otherwise.
3. Scan the whole area in every mode.Metal, live cable and stud, horizontally and vertically, over an area considerably larger than the hole. Mark everything with masking tape, including uncertain readings, and then scan again from a different starting point to see whether the same things show.
4. Reason about the building.What is on the other side of this wall — a kitchen, a bathroom, a boiler, a consumer unit? What is directly above and below — a radiator, a socket, a light switch, a soil pipe? Services run between those points, and a sketch of what is on both faces predicts more than a detector finds.
5. Look from wherever you can.Into the loft above the wall, under the floor below it, behind an adjacent removable panel, through an existing socket box with the circuit isolated, or into the void behind a bath panel. Every one of these tells you more than any scan.
6. Cut a small inspection hole inside the waste.A 30 to 50mm hole, drilled or cut by hand within the area that is coming out anyway, positioned where the scan said nothing was. Hand tools only for this one — a padsaw, a hole saw turned by hand, or a series of small drilled holes.
7. Actually look inside it.Torch and a mirror, or a cheap inspection camera on a phone. Look for cables, pipes, insulation type, the depth of the void, whether the wall is a single skin or has a masonry wall behind it, and where the studs and noggins are. This is the step that turns guesswork into knowledge.
8. Isolate the circuits before enlarging.Switch off the lighting and socket circuits serving that part of the house at the consumer unit. It costs you a torch-lit half hour and removes the electrical risk entirely for the part of the job where a blade is moving.
9. Cut the opening in stages, checking as you go.Extend the hole gradually rather than plunging the full outline. Set the blade depth to just past the board thickness so a multi-tool cannot reach into the void, and stop to look every time the opening grows.

Tips

• An inspection camera that plugs into a phone costs very little and is one of the genuinely useful cheap tools. It pays for itself the first time it shows you a cable you were about to cut.
• Existing socket and switch boxes are ready-made inspection hatches. With the circuit isolated, pulling the faceplate forward shows you the wall build-up, the void depth and the cable route.
• If the wall is dry-lined masonry, the void is often only 10 to 25mm deep and services can be chased into the masonry behind. A short blade and a shallow depth stop matter more here than anywhere.
• Photograph everything you find before you cover it up, with a tape in shot for scale. It is the most useful thing you can leave for the next person, including future you.

Common mistakes

• Plunge-cutting the outline of the opening in one go — A multi-tool blade goes through cable, pipe and board equally happily, and by the time you feel it, it has cut through. Depth-limit the blade and extend the hole in stages.
• Treating a clear detector scan as the whole inspection — The detector's blind spots — plastic pipe, dead cable, foil-backed board, depth — are exactly the things an inspection hole reveals in five minutes.
• Cutting an opening in a wall without establishing whether it is load-bearing — A wall taking load needs a lintel and a proper propping sequence, and it is building control work. Discovering this after cutting is expensive and potentially dangerous.
• Not looking in the loft or under the floor — Both usually show you exactly where the services drop into the wall, for free, before anything is cut.

Questions people ask