Note which side of the house took the weather
Sideways rain can get beneath laps that cope with vertical rainfall.
Movement can be sideways
A slate can shift enough to lose cover without visibly dropping.
Useful evidence: keep one wide photograph that shows where the problem sits on the roof. Close-ups make more sense when the position is obvious.
Avoid sealing drainage gaps at random
Roof details depend on overlap. Closing the wrong gap can trap water.
Want a roofer to look at it?
Describe what you can see and the weather in which the problem appears. That gives the roofer a useful starting point.
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