How do I find pipes and cables before drilling?

The safe zones, what a detector can and cannot find, and how to reduce the risk when you must drill.

Difficulty: beginner
Time: 15 min
Read: 3 min
Safety: warning

Short answer

Use a multi-detector, and stay out of the zones directly above, below and beside sockets and switches — cables usually run there. Neither is a guarantee, so drill shallow first and increase depth gradually.

Cables and pipes are supposed to run in predictable places, and in a well-wired house they do. In an older house, or one with previous DIY, they may run diagonally, at any depth, anywhere. That is why detection and safe zones are both needed and neither is sufficient alone.

Safety

Drilling into a live cable can be fatal and drilling into a pipe causes serious water damage. A detector reduces the risk but does not eliminate it — no detector finds everything, particularly in old properties where wiring may not follow the expected routes. If you hit something, stop, turn off the supply at the mains and get it assessed.

What you'll need

Multi-detector — metal, live cable, stud
Torch (optional)
Masking tape (optional)

Step by step

Learn the safe zones.Cables should run vertically or horizontally in line with a socket or switch, and within 150mm of the top of a wall or an internal corner. Avoid those bands entirely.
Assume the zones are a guide, not a guarantee.In older houses and after previous DIY, cables run diagonally and at odd depths. Treat the zones as places definitely to avoid rather than everywhere else as definitely safe.
Use a multi-detector and calibrate it properly.Follow the instructions — most need calibrating on a clear section of wall first. Sweep slowly in both directions; a fast sweep misses things.
Check in several directions.Sweep horizontally and vertically across the area, and a good margin around where you intend to drill. Mark what you find with tape.
Know what detectors miss.Plastic water pipes are invisible to most detectors. Cables that are not live may not show on live-cable detection. Deep services may be out of range.
Think about what is on the other side.A kitchen on the far side of the wall means pipes. A bathroom above means a waste pipe. Reasoning about the layout catches things detectors do not.
Drill shallow first, and increase gradually.Set a depth stop. Go in a few millimetres at a time rather than driving through in one movement — it gives you a chance to stop.
If you hit something, stop immediately.Do not pull the drill out and carry on elsewhere. Turn off the electricity or water at the mains and get it assessed.

Tips

Turn the power off at the consumer unit before drilling into a wall where you suspect cables. It does not stop damage but it removes the electrocution risk.
Photograph walls before plastering during any building work. It is the only truly reliable record of where things run.
Cheap detectors are unreliable. This is a tool where the better version is genuinely worth the difference.

Common mistakes

Trusting the safe zones alone — They describe how it should have been wired, not how it was. Older and previously modified properties frequently do not comply.
Assuming a detector finds everything — Plastic pipes are invisible to most, and depth limits apply. Detection reduces risk rather than removing it.

Questions people ask

Where do cables usually run in a wall?

Vertically or horizontally in line with a socket or switch, and within 150mm of the ceiling or an internal corner. Avoid those bands — but do not assume everywhere else is clear.

Can a detector find plastic water pipes?

Most cannot. Reason about the layout instead — what is on the other side of the wall and what is above and below — and drill shallow first.