Mold on the roof

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It’s likely a condensation issue rather than a roof leak. Warm, moist air from the bathroom is rising into the attic and condensing on the cold roof sheathing, leading to mold growth. Checking can help assess ventilation concerns. Ensure the bathroom vent is properly sealed and vented outside, improve attic ventilation, and consider adding insulation in that area. The frost and icicles on nails suggest excess humidity, which improved airflow should help resolve.