What is roof mold, and is it the same as roof molding?
Short answer. What people call roof mold on the outside is almost always algae (black streaks), moss or lichen, not true mold; real mold on a roof grows inside, on the attic side of the decking, when warm moist air condenses there. Roof molding is unrelated: it is trim, such as crown molding at the eaves or the metal drip edge along roof edges.
Outside: usually not mold
- Black or dark streaks: algae, typically a blue-green type that feeds on moisture and minerals in shingles. Mostly cosmetic.
- Green, fuzzy cushions: moss, which holds water and can lift shingle edges.
- Flat gray-green or crusty patches: lichen, a partnership of fungus and algae that grips granules tightly.
Inside: true mold
Mold on the underside of the roof deck looks like black, gray or white spotting on the OSB or plywood, often heaviest on the north side and near the ridge. It means moisture is condensing in the attic. Common causes are:
- Bathroom or kitchen fans venting into the attic instead of outside.
- Blocked soffit vents, often from insulation pushed against them.
- Too little exhaust at the ridge, or mixed vent types short circuiting each other.
- A slow roof leak keeping one area wet.
Fixing the moisture source matters more than cleaning the stains. Mold that has weakened the decking is replaced when the roof is replaced.
Roof molding
Some searchers mean trim. Crown molding is sometimes used at the top of exterior walls under the eaves, and drip edge is occasionally called roof edge molding. Neither is related to mold.
Health note
If you see extensive mold in the attic, avoid stirring it up and talk to a qualified remediation professional. Read roof ventilation basics and black streaks and algae on roofs.