Roof cleaning problems

Black streaks. ARMA identifies the most common cause as gloeocapsa magma, a blue-green algae that protects itself from UV by producing a dark pigmented sheath — which is the streak you see. It is primarily an appearance problem. That matters, because roofers sometimes sell staining as structural urgency. It usually is not.

Moss. This one is real damage. Moss is a plant with structure that holds water against the shingle and can lift shingle edges over time, creating a path for water. Shaded, north-facing, tree-overhung slopes get it first. Dealing with moss is genuinely worth doing.

Granule loss. This is the damage caused by bad cleaning rather than by growth. Granules are the shingle's UV shield. ARMA warns that scrubbing with a brush or broom loosens and removes them and that pressure washing will cause granule loss and very likely premature failure of the roof system. If you find a pile of granules at the downspout after a cleaning, that is your evidence.

Recurrence. ARMA is direct that cleaning results are only temporary and discoloration will likely recur. If your roof restains within a year, you were not cheated — that is the nature of the problem. Address the conditions: trim overhanging branches, improve airflow, and consider zinc or copper strips or, at re-roof time, algae-resistant shingles.

How to choose

Separate cosmetic from structural

Streaking is largely cosmetic. Moss is not. Do not let one quote treat them as the same emergency.

Look for granules in the gutters after any cleaning

A visible layer of granules in the gutters or at the downspout outlet after a wash means the method was too aggressive. Photograph it and raise it immediately.

Fix the conditions, not just the growth

Overhanging branches and permanent shade are the cause. Trimming back does more for recurrence than any treatment on the market.

Treat lichen expectations realistically

Lichen bonds tightly to the granule surface. Aggressive removal takes granules with it. A good contractor will tell you some staining may remain rather than promise a spotless roof.

Do not let cleaning substitute for a real inspection

If you have moss lifting shingle edges, you may already have a water path. That is a roofer's question, not a cleaner's.