Start with the conditions
If the stain appears only in wind-driven rain, say so. If it takes half a day of steady rain before anything shows, that matters too. The timing often separates a lap problem from something that has to become saturated before water reaches the room.
Use the loft carefully
From boarded access, look above and uphill from the mark. Fresh wetness on a rafter, damp insulation or a dark line on the underlay can be much more useful than the position of the ceiling stain.
Do not patch the inside and forget the roof
Sealant or foam beneath the covering can divert a drip without restoring the proper weathering outside. The permanent repair should deal with the route rain is taking through the roof.
Allow for drying
Old plaster, timber and insulation may stay damp for a while after the external fault is fixed. A fresh drip during new rain is different from a dark stain that is slowly drying.
Want the roof looked at?
Describe what you have actually seen and the weather in which it happens. That gives the roofer a useful starting point.
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