What Are the Black Streaks on My Roof?
The black streaks on your roof are Gloeocapsa magma — a blue-green algae that feeds on the limestone filler in asphalt shingles. It spreads down the roof with rainwater, holds moisture against the shingles, and shortens roof life. Soft washing kills it at the root; pressure washing damages shingles and voids warranties.
What causes black streaks on a roof?
Gloeocapsa magma arrives as airborne spores, lands on your roof, and starts eating. Modern asphalt shingles contain crushed limestone as filler, and that limestone is food. The algae grows a dark protective coating against sunlight — that's the black you see. Because spores wash downhill with rain, the streaks run vertically, usually worst on the north-facing slope where the sun can't dry things out.
The Shoals is prime territory for it. Humidity off the Tennessee River, warm springs, and shaded lots give the algae everything it needs. Once one roof on a street has it, spores drift to the neighbors — which is why you often see whole blocks with matching streaks.
Do black streaks damage the roof, or just look bad?
Both. The algae holds moisture against the shingle surface, degrades the granules that protect the asphalt underneath, and in the worst cases lifts shingle edges. Left alone for years, it shortens the working life of the roof — and a roof replacement in our area runs five figures. The streaks also read as 'old roof' to anyone looking at the house, which matters if you ever plan to sell.
How do professionals remove black streaks?
Soft washing — a low-pressure application of a cleaning solution that kills the algae at the root, followed by a controlled rinse. No pressure washer ever touches the shingles. This is the method asphalt-shingle manufacturers recommend; high-pressure washing strips the protective granules off shingles and can void the warranty. Anyone who offers to pressure wash your shingle roof is offering to damage it.
Because the treatment kills the organism rather than just knocking off the visible growth, the roof stays clean for years rather than months. We also pre-wet and rinse the landscaping below and manage the runoff, so what comes off the roof doesn't end up killing your shrubs.
Can I remove roof streaks myself?
We'd talk you out of it, honestly. Walking a pitched roof with a sprayer is genuinely dangerous, a rented pressure washer will take granules off with the algae, and getting the solution mix wrong either does nothing or kills the grass below. Roof work is the one job where hiring it out is clearly the right math: professional roof washing starts from $400 — a fraction of one repair bill, let alone a replacement.
Questions we hear about this
Will the black streaks come back after cleaning?
Eventually — the spores are airborne, so no cleaning is permanent. But because soft washing kills the algae at the root, most roofs stay clean for two to three years, longer without heavy shade. Pressure washing, by contrast, leaves the roots alive and the streaks return within months.
How much does it cost to remove black streaks from a roof?
Professional roof soft washing starts from $400, priced by roof size and pitch. In the Shoals you can get an exact number for your home in about a minute with our instant quote.
Is roof cleaning safe for my shingles?
Soft washing is — it uses low pressure and lets the solution do the work, which is the method shingle manufacturers recommend. High-pressure washing is not safe for shingles: it strips granules and can void your warranty.
Why is the north side of my roof worse?
The north slope gets the least direct sun, so it stays damp longest after rain and dew — and damp is exactly what Gloeocapsa magma needs. In shaded, humid areas like the Shoals, north-facing slopes often streak years before the rest of the roof.