Start with the weather
Write down whether the mark appears in steady rain, sudden downpours or only when the wind is blowing from one side. That small bit of history often narrows the search better than staring at the ceiling.
If the leak is intermittent, take a photograph of the stain and mark its edge lightly in pencil. You can then see whether fresh rain is really making it larger.
Look uphill, not just straight above
On a pitched roof, water can run along felt, timber or masonry before it drops. From safe loft boarding, inspect above and uphill from the damp area. Fresh wetness on a rafter, rusty nail points or a darkened patch of underlay are useful clues.
Do not repair the first thing that looks old
A weathered tile, old pointing or a rough-looking piece of lead may be nearby without being the source. The repair should follow the route of the water. Once the external fault is corrected, soaked plaster and insulation can take time to dry.
Want a roofer to inspect it?
Describe the symptom and the weather conditions in which it appears. A few photographs from safe ground can help.
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