Look at the pattern
Condensation often appears on many rafters, nails and areas of felt at once.
Check extractor ducts
A bathroom duct that has come loose can pump warm damp air straight into the loft.
Compare wet and dry weather
If the moisture appears on cold dry mornings, condensation is more likely.
Want somebody to look at the problem?
If you can take a photograph safely from the ground, include one that shows the whole area of roof.
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