What causes attic mold?
Short answer. Attic mold grows when moisture stays on wood long enough, usually from bath or dryer exhaust dumped into the attic, warm house air leaking up and condensing on a cold roof deck, roof leaks, or ductwork sweating in humid summer heat. Weak or unbalanced ventilation lets that moisture linger instead of carrying it out.
The common sources
- Exhaust in the wrong place: bath fans that end in the attic, crushed or disconnected ducts, or outlets right next to soffit intake vents. Mold often shows as a dark patch centered over a bathroom.
- Winter condensation: house air leaking through light fixtures and top plates condenses on the cold north-facing deck. Look for darkening across a whole slope.
- Roof leaks: localized stains at pipe boots, chimneys, valleys and skylights, often with soft wood around them.
- Summer duct sweating: Georgia attics with air handlers and cold supply ducts sitting in very humid air can drip from poorly insulated ducts and boots onto insulation and drywall.
- Blocked intake: buried soffit vents leave stagnant pockets along the eaves, where mildew tends to start first.
What to do
Fix the moisture source before cleaning, or the growth comes back. Light surface mildew on sheathing is common and often cosmetic once the moisture stops; heavy growth, soft decking, or a musty smell inside the house deserves a professional mold assessment. If the roof is being replaced, damaged decking can be swapped while it is exposed. Do not simply add an attic fan: without enough intake it can pull more humid air up from the house.
How to tell which source is yours
The pattern tells the story. A dark patch around a fan duct points to exhaust. Even staining across one slope points to condensation and air leaks. A drip-shaped stain below a penetration points to a leak. See roof ventilation basics and common roof leak causes.