Back to Blog Maintenance Black Streaks on Your Roof? That's Algae. Here's How to Handle It
Mark WhiteOctober 22, 20254 min read If your San Antonio roof has dark streaks running down from the ridge, you're looking at Gloeocapsa magma, a blue-green algae that feeds on the limestone filler in your shingles. It's extremely common in humid climates, and San Antonio's moisture level is just right for it to thrive.
The streaks are ugly, but is the algae actually damaging your roof?
Does Algae Actually Damage Shingles?
Yes, but slowly. Algae colonies hold moisture against the shingle surface, accelerating granule loss and UV degradation. A severely algae-covered roof can lose 2-5 years of useful life compared to a clean one. The damage is cosmetic at first, then structural over time.
The bigger immediate impact is curb appeal and home value. A roof covered in dark streaks looks old and neglected, even if the shingles are only 8 years old. If you're thinking about selling, algae streaks can subtract perceived value.
How to Clean Algae Off Your Roof
The recommended method is soft washing: a low-pressure application of a 50/50 bleach and water solution (with a surfactant to help it cling). Let it sit for 15-20 minutes, then rinse gently with a garden hose.
What NOT to do: never pressure wash your roof. High-pressure water strips granules off shingles and can drive water under the shingle layer. Pressure washing causes more damage than algae does.
Professional soft washing costs $300-$600 for a typical San Antonio home. It's worth having a professional do it, the bleach solution and roof walking involved create safety risks for DIY.
Schedule a Free Roof Inspection
Not sure about the condition of your roof? We will send a certified inspector to your home at no cost. Same day or next business day availability.
Book Your Free Inspection →Prevention: Algae-Resistant Shingles
When you roof replacement, choose shingles with algae-resistant (AR) technology. These shingles have copper or zinc granules mixed into the surface that inhibit algae growth for 10-15 years. Most premium shingle lines now include AR as standard.
You can also install zinc or copper strips along the ridge, rainwater washes trace amounts of metal down the roof, creating an environment algae can't thrive in.
Questions about algae prevention? Legacy Home Improvements offers free roof assessments. Call (210) 272-7272.
Got a roofing question? Something keeping you up at night? (Besides that mysterious dripping sound?) Email me personally at mwhite@legacytx.com. I read and answer every single one.
MW Mark White
Owner, Legacy Home Improvements · San Antonio since 1999
Mark has been in the roofing business for over 25 years. He personally inspects homes, works with insurance adjusters, and answers every email at mwhite@legacytx.com.
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