Attic mold, explained
What Black Roof Sheathing in Your Westland 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. In winter, warm house air rises into the attic, hits the cold deck and condenses, sometimes freezing into frost on the nail tips that melts and drips through the ceiling on the first sunny day.
The sources are simple once you know where to look. Fan ducts that end in the insulation instead of at an exterior cap, soffit vents choked off by blown-in insulation, a ridge vent that's undersized or was never installed, and air leaks at top plates, can lights, plumbing stacks and the hatch all feed warm, wet air where it can't escape. 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. 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.