What Are the Black Streaks on My Roof? (Cape Coral Roof Algae Explained)
Short answer: the black streaks on most Cape Coral roofs are algae, usually a type called Gloeocapsa magma, not dirt or mold. They spread in warm, humid weather, start on shady slopes, and should be removed with a low-pressure soft wash, never a power washer.
By the Pressure Washing Cape Coral team · Last updated: October 2026
What Are the Black Streaks on My Roof?
The black streaks on your roof are a blue-green algae, most often Gloeocapsa magma. The Asphalt Roofing Manufacturers Association (ARMA) explains that this algae spreads across roofs in warm, humid climates, and its dark outer coating protects it from the sun, which is why it looks black instead of green.
In Cape Coral, the air stays muggy from about April into November, so the algae has a long growing season. That’s why streaked roofs are so common across Cape Coral and Southwest Florida.
Why Do Black Streaks Start on One Side of the Roof?
Streaks usually start on the north side and in shady spots, because those areas stay damp the longest after rain and dew. Roofs under big trees, or next to taller buildings, streak faster. The algae then spreads down the slope with the rain, which is what creates the streaked look.
Are Black Streaks Bad for Your Roof?
Black streaks are mainly a looks problem, but they shouldn’t be ignored. The algae spreads over time, holds moisture on the surface and makes the whole house look older. Cleaning it early is easier than waiting until the roof is covered.
How Do You Remove Black Streaks From a Roof?
You remove black streaks with a low-pressure soft wash: a cleaning solution is applied to the roof, given time to work, then rinsed gently. ARMA advises against using a power washer on asphalt shingles, because high pressure can strip the granules that protect them. On tile, high pressure can crack or chip the tiles.
| Method | Result |
|---|---|
| Low-pressure soft wash | Removes algae without damaging tile or shingles |
| Power washing | Risks cracked tile, stripped granules and water under the roofing |
| Scrubbing | Can damage shingles and is unsafe on a roof |
See how we do it on our Roof Cleaning page.
Will Black Streaks Come Back?
Yes, over time, because the algae spores are in the air and our climate stays humid. How fast depends on shade, nearby trees and water, and the roof. Regular cleaning keeps the streaks from building up again.
Black Streaks FAQ
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