What is the difference between roof moss and algae, and how do you treat both?
Short answer. Algae and moss often show up together, but they are different problems: algae makes flat black or dark streaks that are mostly cosmetic, while moss is a raised green plant that holds water and lifts shingle edges. A combined moss and algae remover can treat both, but moss also has to be physically removed once dead, while algae streaks usually just fade.
How to tell them apart
- Algae: flat dark streaks running down the slope, most common on north and shaded faces. No texture you can feel.
- Moss: raised green or yellow-green cushions, often along shingle edges and in the gaps between tabs. Soft and spongy when wet.
- Lichen: crusty gray-green or pale patches stuck tight to the surface.
Which one does damage
Algae mainly affects appearance, though heavy growth on shaded slopes can hold a bit of moisture. Moss is the damaging one: it keeps shingles wet, lifts the edges, breaks the seal strips that hold shingles down in wind, and traps leaves and pine needles. Roofs with moss usually had algae first, because the roughened, damp surface makes it easier for moss to take hold.
Treating both
- Use a product labeled for both moss and algae on asphalt shingles, or treat moss first and handle algae streaks with a diluted bleach mix as the manufacturer allows.
- Apply on a mild, dry day and protect plants below.
- Wait for the moss to die and loosen, then brush it downhill gently.
- Let rain rinse the algae residue; streaks fade over weeks.
- Never pressure wash.
Prevention
More sunlight, clean valleys and gutters, and zinc or copper strips near the ridge help with both. At replacement time, algae-resistant shingles reduce streaking. See black streaks and algae on roofs and moss and lichen on roofs.