Attic mold, explained
Why Mold Shows Up on the Underside of Your Roof Deck
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. The colder north-facing slope tends to be the worst of it, because that side gets the least sun and stays damp the 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. 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.
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.