How do you tell roof algae, moss, and lichen apart?
Roof algae appears as flat black or dark gray streaks running down the slope. Moss looks like green, fuzzy, grass-like mats that grow upward off the shingle surface. Lichen forms light green, gray, or white crusty patches that grip the shingle tightly. All three thrive in shade and moisture, but they attach differently and must be treated differently.
What are black streaks (algae)?
The black streaks are colonies of Gloeocapsa magma, a cyanobacteria that feeds on the limestone filler in asphalt shingles. It lies flat against the shingle, follows the water path down the slope, and darkens as colonies mature. It is by far the most common of the three growths on asphalt roofs in humid climates.
What does moss on a roof mean?
Moss is a true plant. It grows in thick, spongy green mats, usually on heavily shaded slopes, near overhanging branches, and along the north side of the roof. Moss is more damaging than algae in the short term because it holds moisture directly against the shingles and its root-like structures can lift shingle edges, opening a path for water intrusion and wind damage.
Never Scrape or Pull Moss
What are the crusty patches (lichen)?
Lichen is a symbiotic partnership between a fungus and algae. It forms flat, crusty, plate-like patches that bond aggressively to the shingle surface. Lichen is the most stubborn of the three: it grips so tightly that mechanical removal tears granules off with it. Like moss, lichen must be killed chemically and allowed to release on its own over time.
Do all three get treated the same way?
The kill step is the same: a low-pressure application of a sodium hypochlorite solution, per the ARMA-approved soft wash method. The difference is what happens after. Algae rinses away during the cleaning, delivering an immediate visual result. Moss and lichen die during treatment but must detach naturally over the following weeks. Any contractor promising to make moss or lichen disappear on the same day is planning to remove it mechanically, which damages shingles.
The presence of moss or lichen alongside algae is also one of the signs a roof problem has moved beyond DIY solutions. See the warning signs your roof growth needs professional intervention.