How do roofers find where a roof is leaking?
Roofers usually combine the location and pattern of internal water staining with an external inspection of tiles, slate, flashing, valleys, chimneys, rooflights and drainage details. Loft inspection, moisture patterns, photographs and drone imagery can also help because the point where water appears indoors may not be where it entered.
How the search is structured
Leak tracing works from the inside out. The roofer establishes where water is arriving, looks in the roof space for the highest point of staining or wetness, then works upslope from there to the details most likely to be responsible.
Because water travels, the internal evidence sets the search area rather than pinpointing the defect. The external inspection then examines the covering, flashings and junctions above that area.
Tools that help
Close inspection, loft access and, where it helps, aerial photography of areas that cannot be seen safely from a ladder all contribute. Photographs are useful because they let you see what the roofer saw rather than take it on trust.
Some leaks only show under particular conditions — wind-driven rain from one direction, for example — and those can take more than one visit to confirm.