Attic mold, explained
What Black Roof Sheathing in Your Jamestown West 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. The colder north-facing slope tends to be the worst of it, because that side gets the least sun and stays damp the 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. Ice dams and leaks at flashing or roof penetrations add liquid water on top of that.
That's why cleaning the wood is only half of a real Attic Mold Removal job. Unless the venting and bypasses get fixed, the deck will be stained again in a few seasons, no matter how carefully it was cleaned and coated.