Gutters before a wet spell
Clear heavy debris and make sure outlets run. Overflow at the eaves can soak walls and fascia for hours after rain ends.
Cold lofts
Moisture on felt and nails after a cold night can be condensation. Check extract ducts and blocked ventilation before looking for a hole in the roof.
Wind damage
Look for changed slate lines, ridge pieces out of position and fragments on the ground. Leave close inspection until conditions are safe.
Freeze and thaw
Open mortar joints can deteriorate further when saturated material freezes. Loose high-level pieces deserve more attention than cosmetic surface weathering.