Look for a repeatable weather pattern
Note whether staining changes during rain, several hours afterwards, or only when wind comes from a particular direction. Water can take time to work through the roof space and ceiling, so the patch may appear after the rain has stopped.
Record the location and dates using photographs taken from a safe position. Wind-driven rain can behave differently from a short shower. A long-standing mark that stays unchanged might be old damage, although a dry spell does not prove a repair has worked.
Where outside water can get in
On an upper-floor ceiling, possible routes include damaged roof coverings, failed flashing around a chimney or roof junction, and problems around a rooflight. Blocked or defective gutters can allow water to reach parts of a building that should stay dry.
Flat roofs also have vulnerable junctions, outlets and penetrations. Finding the place water enters may take more than looking directly above the internal stain. Water can track along the construction before it becomes visible indoors.
These are possible routes to assess, not a remote diagnosis. Do not climb onto a roof or walk on ceiling joists to check them yourself.
A roof-space problem is not always a rain leak
Condensation can form on cold surfaces in a roof space when moist air reaches them. This may contribute to dampness below even when the roof covering is intact. Ventilation, insulation and the movement of warm moist air can be relevant.
Wet weather can coincide with cooler temperatures and closed windows, which makes a simple rain-versus-dry comparison imperfect. An inspection should distinguish external entry, roof-space condensation and any plumbing in the loft.
Who to contact next
If the link to rain is clear, an appropriate roofing or building specialist can assess the roof and drainage safely. Where the route is unclear, ask ADI whether a water-ingress investigation is suitable for the construction involved.
If the patch also grows in dry weather or after bathroom use, tell the investigator. That information may change the order of checks. Electrical hazards and a sagging ceiling need urgent attention regardless of the weather.