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
"Very professional, excellent job and very accommodating. Couldn’t ask for a better group of young men to work with."
Pamela P. · Google Review
"Like the name of the company 1st class ! From job estimate, to job completion. Great communication, and excellent service. Our house looks brand new. Will be recommending and using them again."
mark W. · Google Review
100 Google reviews from homeowners and businesses across Middle Tennessee. Read them all on Google.
Get a free quote
Get a Free Quote Today
Same-day estimates · No pressure · No obligation
Free quote by voice · 24/7
Skip the form. Just talk.
Tell our AI assistant what needs cleaning. It answers your questions and takes your quote details, any time of day or night.
or fill out the form
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
Serving Williamson County
Franklin · Brentwood · Spring Hill · Nolensville · Thompson's Station · Arrington · College Grove · Belle Meade · Nashville · Murfreesboro
Sources
Rather Have the Pros Handle It?
Call for a free, same-day estimate or send us a few details and we'll get right back to you.