Direct answer
What should you know first?
Black streaks on asphalt shingles are Gloeocapsa magma, a blue-green algae that feeds on the limestone in modern shingles and spreads across shaded roof areas. The safe removal method is soft washing: a cleaning solution applied at low pressure that kills the organism and lets rain rinse the residue. High-pressure washing or scrubbing dislodges the protective granules and shortens the roof's life.
What the streaks actually are
Shingle manufacturers began adding crushed limestone to asphalt shingles decades ago, and the algae that feeds on that limestone arrived with it. It starts on the shaded side of the roof, often under trees, and spreads downward in streaks as rain carries it across the surface.
Beyond appearance, an established colony holds moisture against the shingle and can accelerate granule loss in the areas it covers.
Why pressure is the wrong tool for a roof
Asphalt shingles depend on a layer of ceramic granules for UV protection and fire rating. A pressure washer at cleaning distance lifts those granules off, and the damage is permanent. Walking the roof and scrubbing causes similar wear.
Soft washing reverses the approach: chemistry and dwell time do the work while water pressure stays low, closer to what a steady rain produces.
- Never high-pressure wash asphalt shingles
- Soft washing kills the algae rather than stripping the surface
- Streaks continue to fade for weeks after treatment as rain rinses the residue
What a professional cleaning should include
A proper roof cleaning starts with an inspection for loose, cracked, or missing shingles, since cleaning does not repair those conditions. Plants and shrubs below the roofline should be rinsed and protected, and gutter downspouts checked so the treatment drains where intended.
Expect the darkest streaks to lighten immediately and continue fading over several weeks. A healthy roof typically stays clean for years before growth returns, depending on shade and tree cover.