Moss and Lichen on a Shingle Roof: What to Do
Green clumps and gray crusty patches on the shaded side of a roof are not the same as the black streaks. What roofing makers say about moss, and why a pressure washer is the wrong tool.
The short answer
Moss is the growth roofing makers worry about, because it can hurt how asphalt shingles perform. Black algae streaks look bad but have not been shown to damage shingles. The roofing industry says never to pressure wash a shingle roof. The safe way is a cleaning solution applied at low pressure, given time to work, then a gentle rinse.
Three different things grow on a roof
From the driveway, a roof with growth on it just looks dirty. Up close there are usually three different things.
- Black streaks are algae. They run down the slope in stripes.
- Moss is green and soft, and it grows in clumps, often where the roof stays shaded.
- Lichen looks like flat gray or pale green crusty spots stuck tight to the shingle.
They often show up together on the side of the house that gets the least sun.
Why moss is the one to take seriously
The Asphalt Roofing Manufacturers Association, the trade group for the companies that make shingles, draws a clear line between them. It says “there is no scientific evidence that algae is damaging to asphalt shingles,” but “unlike algae, moss can be detrimental to asphalt shingle performance” (Asphalt Roofing Manufacturers Association).
So black streaks are mostly about looks. Moss is about the roof itself.
Why a pressure washer is the wrong tool
The roofing industry and the shingle makers agree on this. ARMA says to “never use a pressure washer to clean an asphalt shingle roof” (Asphalt Roofing Manufacturers Association). CertainTeed, one of the big shingle makers, says “do not use a power washer on your asphalt shingle roof” (CertainTeed roof care).
High pressure blasts off the granules, the small stones on the surface of a shingle that protect it from the sun.
What the roofing industry says works
ARMA describes a bleach and water solution put on the roof, left to sit for 15 to 20 minutes, then rinsed off “with low pressure water” (Asphalt Roofing Manufacturers Association). The solution does the work, not the water.
That is the same idea as our roof cleaning. We soft wash: low pressure plus a cleaning solution that kills the moss, lichen and algae, instead of scraping or blasting them off.
Why it comes back
Moss and lichen grow where a roof stays damp and shaded. Cleaning does not change that. ARMA’s algae guidance says the effect of cleaning “is only temporary, and the discoloration may recur” (Asphalt Roofing Manufacturers Association). Trimming back branches that shade the roof helps it dry out faster.
ARMA also says that adding zinc or copper strips to an existing roof “is not recommended” (Asphalt Roofing Manufacturers Association), so we do not sell them as a fix.
The rest of the house
A roof with moss on it usually sits above siding with the same green film. See green algae on vinyl siding.
What cleaning will not fix
Cleaning removes the growth. It will not fix shingles that are already curled, cracked or missing granules, and it will not stop growth from coming back on a roof that stays shaded and damp. The roofing industry itself says the effect of cleaning is temporary.
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