What is the difference between moss and mold on a roof?
Short answer. Moss is a small green plant that grows in raised cushions; what people call mold on a roof is usually algae (black streaks) or mildew on debris, which lies flat. Lichen is a gray green crust. Moss causes the most physical damage, while flat algae and mildew are mostly cosmetic.
How to tell them apart
- Moss: green, soft, raised clumps or carpets, usually along shingle edges and in shaded areas. It has tiny root like rhizoids and holds water like a sponge.
- Algae: black or dark green streaks running downslope, flat to the surface. Often mistaken for mold or dirt.
- Mildew or mold: fuzzy white, gray or black growth, usually on leaf piles, pine straw or wood trim rather than clean shingles.
- Lichen: gray, pale green or yellow crusty spots that cling tightly and grow slowly.
What each one does
- Moss: lifts shingles, traps water, and leads to leaks and rot. The most damaging.
- Algae: mostly cosmetic, but keeps surfaces damp and prepares the way for moss.
- Mildew or mold: feeds on organic debris; clearing the debris usually solves it.
- Lichen: bonds to granules; removing it by force pulls granules off.
Treatment differences
- Algae and mildew respond to a gentle bleach soft wash and rinse.
- Moss needs a roof safe moss killer, time to loosen, then gentle downslope removal.
- Lichen needs to be killed and left to weather off, sometimes over months.
- None of them should be pressure washed off asphalt shingles.
Mold inside versus outside
Growth on the roof surface is different from mold on the attic side of the decking, which comes from leaks, poor ventilation or bathroom fans venting into the attic. See roof ventilation basics, black streaks and algae and moss and lichen.