Attic mold, explained
What Black Roof Sheathing in Your Pottsville Attic Really Means
Most attic mold calls start the same way: a home inspector or roofer shines a flashlight through the hatch and sees the sheathing gone gray or black between the rafters. It's usually worst on the north-facing slope, since that side of the roof stays coldest and holds condensation longest. Through the cold months, humid air from the living space drifts up, meets the chilled underside of the deck and turns to water or frost, and that frost on the roofing nails eventually melts onto the insulation and drywall.
The moisture almost always comes from a short list. Bath and kitchen exhaust fans that dump into the attic instead of venting outdoors, soffit vents buried under blown-in insulation, a weak or missing ridge vent, and attic bypasses around top plates, recessed lights, plumbing stacks and the hatch all push wet air into a space with no way out. An ice dam or a leak at the flashing around a vent pipe adds liquid water to the condensation.
This is the reason cleanup alone never settles it. If the airflow and air leaks aren't corrected, the same boards go black again within a few winters, however well they were blasted and sealed.