Inspect the roof above
A split slate, damaged underlay or leaking gutter joint can feed one short section.
Old felt can create a hidden trough
Sagging underlay behind the fascia can carry water sideways before it escapes.
Check the rafter ends
Once the fascia is open, look at the timber behind it rather than assuming the visible board is the only damaged part.
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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