What Are the Black Streaks on My Roof?
Dark streaks running down asphalt shingles aren't dirt or mildew. They're a blue-green algae called Gloeocapsa magma, and in Middle Tennessee's humidity they spread from roof to roof on the wind.
What causes it
The algae feeds on the limestone filler used in many asphalt shingles. It grows fastest on the north and shaded sides of a roof, where shingles stay damp longest. As it grows it builds a dark protective coating, which is the black you see.
Is it hurting the roof?
The algae itself is mostly cosmetic at first. But the dark layer holds moisture and heat, and if moss or lichen move in behind it, their roots can lift shingle edges. The Asphalt Roofing Manufacturers Association recommends removing it with a gentle cleaning solution, not pressure.
Why roofs must never be pressure washed
High pressure strips the ceramic granules that protect shingles from UV, which shortens roof life and can void the warranty. A proper roof soft wash applies a cleaning solution at garden-hose pressure, lets it kill the algae, and rinses off.
What to avoid
- Pressure washers of any kind on shingles.
- Walking on a wet, algae-covered roof. It's extremely slippery.
- Leaving runoff on plants. Pre-wet and rinse landscaping.
When to call a professional
Roof work involves heights, slick surfaces and chemicals near landscaping, so it's a job for insured pros. A soft-washed roof typically stays clean for several years. See our roof cleaning service.
What customers say
"First Class Cleaning washed our entire church building roof to ground. The building had built up quite a bit of algae and mildew over the years. It now looks brand new. The guys worked with our preschool and ministry schedule. They got the job done quickly and at a fair price."
Henry H. · Google Review
"I used First Class Pressure Cleaning to seal my aggregate driveway and they did a fantastic job. It was in rough shape as we had never sealed it before. Lots of salt damage as well from trying to de-ice the last winter storm. It looks so much better now. They even came back the next day for touch-ups on some areas where the winter salts had worn through to the concrete. Very happy with this company and the work they’ve done!"
Dan · Google Review
"Amazing service and extremely professional!!! All looks amazing :)"
Jennifer B. · Google Review
96 Google reviews from homeowners and businesses across Middle Tennessee. Read them all on Google.
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Related guides
- Moss on Your Roof: Why It Grows and How to Remove It Safely
- How to Clean a Metal Roof
- How to Get Rid of Green Algae on House Siding
- How to Clean a Green or Dirty Vinyl Fence
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