How do I find where a water leak is coming from?

Tracing a leak back to its source, including why the wet patch is often nowhere near the actual problem.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 3 min
Safety: caution

Short answer

Water travels along joists, pipes and cables before it drops, so the stain is often well away from the source. Work out whether it appears constantly, only when water is used, or only after rain — that alone narrows it to one system.

The single most useful diagnostic is timing. A constant leak is a supply pipe under pressure. A leak only when water is used is a waste pipe. A leak only after rain is the building fabric. Each points to a completely different search.

Safety

Turn off the electricity at the consumer unit before investigating any leak near lights, sockets or a ceiling containing wiring. If water is coming through a ceiling, treat the area as unsafe until the circuit is isolated.

What you'll need

Torch
Kitchen paper or dry cloth
Water meter access (optional)
Moisture meter (optional)

Step by step

Note when the water appears.Constant means a pressurised supply pipe or heating pipe. Only when a bath or washing machine drains means a waste pipe. Only after rain means the roof, gutters or walls.
Check the whole area above and around the stain.Water runs along joists and pipes before dropping. The source can easily be metres away from where it becomes visible.
Dry the area and lay kitchen paper on it.It shows exactly where and when water arrives, and how fast. This is the cheapest and most informative test available.
Use the water meter to test for a hidden supply leak.Turn off every tap and appliance, note the meter reading, wait an hour without using water. Any movement means a leak somewhere on the supply.
Isolate systems one at a time.Turn off the heating for a day, or the water at the stopcock. If the leak stops, you have identified which system is at fault.
Check the obvious culprits.Shower seals, bath sealant, toilet connections, washing machine hoses, radiator valves, and the overflow pipe outside.
For rain-related leaks, inspect during or straight after rain.Gutters overflowing, a slipped tile, blocked downpipes, failed pointing, or a blocked weep hole. It is much easier to see while it is happening.
Distinguish a leak from condensation.Condensation forms on cold surfaces in humid rooms, appears in cold weather, and is spread over an area rather than concentrated. Leaks are localised and often worsen.

Tips

Photograph the stain with a date. Whether it grows over days tells you whether it is active or historic.
A moisture meter is inexpensive and lets you trace a damp area back to its wettest point, which is usually towards the source.
If you cannot find it, a leak detection service uses thermal imaging and acoustic equipment. Many insurers cover the cost of tracing.

Common mistakes

Assuming the leak is directly above the stain — Water runs along joists and pipes for metres before dropping. Searching only overhead misses most leaks.
Redecorating before the leak is fixed — The stain returns and the new work is wasted. Fix, dry thoroughly, then decorate.
Ignoring a small stain — Sustained damp rots timber and causes mould. Small persistent leaks do far more cumulative damage than dramatic ones.

Questions people ask

How do I know if it is a leak or condensation?

Condensation appears on cold surfaces, in cold weather, spread over an area, often with black mould. A leak is localised, can appear in any weather, and often correlates with using water or with rain.

How can I check for a hidden water leak?

Turn everything off, note your water meter reading, and wait an hour without using any water. If the reading has moved, there is a leak on the supply side.