Inspect uphill from the damaged timber
A split slate, poor underlay or leaking gutter joint can feed one local area.
Old felt can sag behind the fascia
It may collect water and discharge against the timber rather than into the gutter.
Useful evidence: if you can take a photograph safely, include enough of the roof to show where the defect sits. A close-up without context can be surprisingly unhelpful.
Check rafter ends while the fascia is open
Surface staining and structural decay are different things and should not be treated the same.
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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