Start with what is directly above the stain
Water through a ceiling is alarming, but the wet patch does not always sit directly under the failed fitting. Water can run along joists, pipes or the back of plasterboard before it finds a low point. Think about the room above first. A bathroom, airing cupboard, radiator or pipe run makes a plumbing leak more likely. If the stain only appears after rain, the roof may be the better place to investigate. If it grows after somebody showers, flushes the toilet or empties a bath, that timing is a valuable clue.
Different plumbing faults leave different clues
A leaking bath waste often shows itself when a large volume of water is released, while a failed shower seal may only cause trouble during use. A small leak from a pressurised pipe can wet the ceiling at any time, including overnight. Toilet cisterns and supply pipes can leak slowly enough to soak surrounding timber before water finally becomes visible downstairs. Central-heating pipes may leave rusty or slightly coloured marks rather than a clean water stain. Looking at when the patch changes is often more useful than judging the colour of the plaster alone.
What to do while you are finding the source
Keep people away from any bulging or heavily saturated part of the ceiling. If water is close to a light fitting or other electrical point, switch off the relevant circuit if you can do so safely and do not touch wet electrical equipment. Stop using the bath, shower, toilet or appliance above if it seems to trigger the leak. If the water continues with everything upstairs left unused, the fault may be on a supply pipe and the mains stop tap may need to be turned off until help arrives. Do not cut into a ceiling simply to search for the leak.
Why the visible damage can spread quickly
Plasterboard loses strength when it becomes saturated, and insulation can hold a surprising amount of water out of sight. Chipboard flooring in the room above may swell, while timber can remain damp long after the drip has stopped. This is why a slow leak should not be dismissed merely because only a small brown mark is visible. The plumbing repair may be modest, but leaving the source unresolved can turn it into a decorating, flooring and ceiling repair as well. Drying the area properly after the leak is fixed matters just as much as stopping the water.
How a plumber may trace it
A plumber usually starts by asking what was being used when the water appeared. They may test individual fixtures one at a time, check accessible pipework and look around bath panels, toilet connections or radiator valves. Moisture meters can help show where materials are wet, although they do not identify the cause by themselves. Sometimes a small access opening is unavoidable, but there is no value in making a large hole before the likely source has been narrowed down. The best investigation follows the pattern of use rather than randomly opening finishes.
What to tell contractors before they quote
Describe the room above the leak, when the ceiling first became wet and whether the mark changes after a particular fixture is used. Say if the water is still actively dripping or whether it has stopped. Photographs of both the ceiling and the plumbing above can help. If there is any electrical involvement, make that clear before the visit. A contractor quoting from limited information may sensibly give an investigation charge first and a repair price once the actual source is found; for hidden leaks, that is often more realistic than pretending the diagnosis is certain from a photograph.
Before repairs to the ceiling begin
Do not rush to redecorate as soon as the dripping stops. The plumbing fault should be repaired and the affected area given time to dry before plaster, paint or damaged boards are replaced. If the leak came from a bathroom, test the relevant fitting again after the repair while somebody watches the ceiling below. That simple check can reveal a second weak seal that was hidden by the first leak. Where a large area has been soaked, ask whether insulation or damaged plasterboard needs to be removed rather than left damp in the void. The plumber may only be responsible for stopping the water, so establish separately who will deal with drying, ceiling repairs and decoration if those jobs are necessary.
This page is general guidance about water coming through ceiling; the cause in an individual home still needs to be established from the actual installation.