Start with what you can see
Where is the water coming from?
Choose the symptom closest to yours. The links lead to possible causes and specific checks, not an instant diagnosis.
Find the most relevant guides
Water appearing on the ceiling
Possible starting points, not a diagnosis:
Damp worse after rain
Possible starting points, not a diagnosis:
Water around windows or doors
Possible starting points, not a diagnosis:
Wet walls low down
Possible starting points, not a diagnosis:
Check safety first
If water is near electrics, a ceiling is sagging, sewage is entering the house, or there is a rapid leak, deal with the immediate risk and arrange appropriate professional help. Do not access roofs or unsafe voids yourself.
Record the pattern
Photograph the position, note when it appeared and whether it changes after rain or use of water fittings. This information will help a competent investigator narrow the possibilities.