Local decay is useful evidence
If most of the fascia is sound and one section is badly affected, inspect the roof above. A split covering, leaking gutter joint or damaged underlay may be feeding it.
Old felt can form a hidden trough
Sagging underlay behind the fascia can collect water and carry it sideways. Replacing the board without correcting that route leaves the new material exposed.
Check the rafter ends
Once the fascia is open, inspect the timber behind it. Surface staining and structural decay are not the same thing.
Want a roofer to inspect it?
Describe the symptom and the weather conditions in which it appears. A few photographs from safe ground can help.
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