Attic mold, explained
What Black Roof Sheathing in Your Agua Dulce Attic Really Means
Plenty of homeowners never knew they had attic mold until an inspector or roofer climbed the ladder and found the deck boards stained 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. 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.