Assuming there is no roof leak, investigate: 1) wind-driven rain or snow through attic, gable, or soffit vents; 2) if ductwork is in the attic and it's wet in the summer, have the ducts checked for leaks and sealed; 3) if it's wet in winter, it might be condensation from insufficient attic ventilation, air leaks from ceiling penetrations, or lack of a vapor retarder in the ceiling (cold climate); 4) exhaust vents carrying moisture from the bathroom, kitchen, or moist utility areas could be directly vented in the attic - all exhaust vents should be vented to the outside.

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