Clear debris first
Leaves, moss and mortar fragments can hold water against the edge of a valley.
Trace the full run
Water seen low down may have entered near the ridge or another junction.
Different linings need different repairs
Lead, preformed channels and tiled valleys should not all be treated with the same surface coating.
Want the roof looked at?
Describe what you have actually seen and the weather in which it happens. That gives the roofer a useful starting point.
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