How does a roofer find the actual source of a roof leak?

When water appears is evidence. Heavy rain only, wind from one direction, after rain has stopped, or in cold weather without rain each point in a different direction. Five minutes of questions frequently removes half the possible causes.

Previous repairs matter as much. Knowing what has already been done, and whether the leak changed afterwards, tells us whether the earlier diagnosis was wrong or incomplete.

The roof space shows the route. We look for the highest point where underlay, timber or insulation is wet or stained, because water runs downwards and sideways from where it entered but never upwards. That point sets the search area on the roof above.

Insulation, cabling, tank overflows and ventilation are all checked at the same time, because not every damp roof space is a leaking roof.

Externally, the details within and above the search area are examined closely: coverings, laps, valleys, flashings, abutments, penetrations, ridge and verge. Photographs are taken so each observation can be located and discussed rather than described from memory.

Where access is difficult, a visual drone survey covers whole elevations quickly and identifies where physical access is actually needed, which usually reduces the access cost rather than adding to it.

If the route is still unproven, water is applied deliberately to one area at a time, starting at the lowest point and working upwards, with someone watching inside. It is slow and it must be done in that order, otherwise a positive result cannot be attributed to one area.

A leak that has defeated several repairs almost always needs this. It is also the point at which an honest contractor says the cause is not yet established rather than proposing a replacement.