Mark the stain before it dries
A pencil mark around the edge or a dated photograph helps show whether the damp area is growing. This is useful if the leak is intermittent.
Check the loft safely
If there is safe access, look for wet rafters, darkened battens or water on the underlay above and uphill from the stain. Do not step between joists onto plasterboard.
Think about roof features nearby
Chimneys, valleys, roof windows and changes in roof pitch all create possible entry points. A plain patch of ceiling may be below one of these even if it is not obvious from the room.
After the repair
Give the ceiling time to dry before redecorating. If insulation is saturated, it may need lifting or replacement so moisture is not trapped against the plasterboard.
