Why is my roof leaking?
Common causes of roof leaks include slipped or broken tiles, failed flashing, deteriorated valleys, damaged flat-roof coverings, problems around chimneys and rooflights, blocked drainage and storm damage. Water can travel along timbers before appearing indoors, so the internal damp patch is not always directly beneath the entry point.
Why the leak is rarely above the wet patch
Water entering a roof follows the easiest path down. It can run along the underside of underlay, across battens and along rafters before it finds a point to drip, which is why the mark on the ceiling and the defect on the roof are often metres apart.
This is the single biggest reason DIY leak hunting fails. Sealing the nearest visible gap frequently changes where the water appears without stopping it coming in.
The details that fail most often
Junctions do most of the work on a roof and cause most of the leaks: chimney and abutment flashing, valleys, roof window surrounds, and the point where a lower roof meets a wall.
Slipped or cracked coverings, failed pointing at ridge and verge, and blocked gutters that let water back up under the bottom course are the other common culprits. A methodical inspection works through those in order rather than guessing.