Your Roof Shingles May Have A Microscopic Pest that Leaves a Mess: Blue-Green Algae

By Don Kilcoyne 01-26-2022
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If you drive down virtually any residential street in the U.S. or Canada, there's a chance you'll see at least one roof stained with black or dark brown streaks. Many assume the stains on their roof shingles are from mold, mildew, moss, or even tree sap. They're often actually caused by tiny bacteria called Gloeocapsa magma, AKA "blue-green algae". When you see the dark stains, you're not looking at the algae, but at the hard, dark coating it generates to protect itself from damaging UV rays.

The difference between moss and blue-green algae is where it comes from, as well as how to treat it. While moss is a plant, and can do damage to your shingles, blue-green algae is a different story. Gloeocapsa magma (blue-green algae) travels on the wind — which means if your neighbor has it, your roof has probably been exposed as well. It thrives in moist environments and can get nutrients from dust in the air. It is more likely to appear on the north and west sides of the house. According to the Asphalt Roofing Manufacturers Association (ARMA), there's no scientific evidence that algae is damaging to asphalt roof shingles. However, it can definitely affect the appearance and resale value of the home. And by darkening the roof, it may negatively impact the performance of highly reflective cool roofs. That's the bad news. The good news is that blue-green algae is both treatable and, if you plan ahead, preventable.


A number of commercially available cleaning solutions will help remove the stains and impede algae growth for a year or more. GAF recommends cleaning the roof with a special mixture: 4 gallons (18.2 L) of water, 1 gallon (4.5 L) of bleach, and 1 cup (236 mL) of TSP (Tri-Sodium Phosphate-or acceptable phosphate-free substitute) found at many home centers. Apply this mixture with a garden sprayer, let it sit on the roof surface for up to 20 minutes, and rinse with low-pressure water. Be sure to protect any shrubs below. Here are a few tips to keep in mind:

  • As with any roofing project, use appropriate safety gear and follow best practices.

  • Be very careful on the roof after applying any cleaning solution, as it may make the roof slippery.

  • When working with bleach, wear rubber gloves and eye protection.

  • Be sure to apply the solution carefully, to avoid damage to other parts of the building and surrounding landscape.

  • Do not use high-pressure washing systems on asphalt roofing, as they can dislodge granules and damage the roof shingles.


Certain metal ions, like copper and zinc, can help prevent algae growth. Copper and zinc strips are sometimes installed at the roof ridge, where rainwater will pick up the algae-preventing ions and distribute them on the roof. This is not the most effective method since algae do not rely on rain to find moisture. In fact, it can feed on the moisture from humidity and morning dew.

GAF shingles that have Time-Release Algae-Fighting Technology deliver the algae-fighting copper in a whole new way. Instead of a copper layer, specially-engineered capsules are infused throughout with copper microsites, which release copper steadily over time, for long-lasting algae-fighting power. It's algae resistance is so powerful it allows GAF to offer a 25-year StainGuard Plus™ Algae Protection Limited Warranty* against blue-green algae discoloration on many of GAF's most popular products including, Timberline® UHD Lifetime High Definition® Shingles and Timberline® HDZ. Click here to learn more about GAF Time-Release Algae-Fighting Technology, the 25-year StainGuard Plus™ Algae Protection Limited Warranty* and qualifying products.

Regular roof maintenance can also help discourage algae growth:

  • Trim tree branches back to allow more sunshine and reduce debris.

  • Clear existing debris from the roof with a leaf blower or other non-abrasive method.

  • Keep gutters clean to avoid water build-up.

If you're going through the effort and expense of installing a beautiful new roof, it makes sense to help protect that investment against ugly blue-green algae stains by choosing GAF shingles with Time-Release Algae-Fighting Technology and the 25-year StainGuard Plus™ Algae Protection Limited Warranty*. To review your specific roofing needs, reach out to a contractor certified by GAF** in your area.

*25-year StainGuard Plus™ Algae Protection Limited Warranty against blue-green algae discoloration is available only on products sold in packages bearing the StainGuard Plus™ logo. See GAF Shingle & Accessory Limited Warranty for complete coverage and restrictions.

**Contractors enrolled in GAF certification programs are not employees or agents of GAF, and GAF does not control or otherwise supervise these independent businesses. Contractors may have agreed that they will use GAF roofing products, and may receive benefits, such as loyalty rewards points and discounts on marketing tools from GAF for participating in the program.

Don Kilcoyne, a writer and editor for GAF, crafts marketing campaigns and language that communicate the company brand, initiatives, products, and priorities in video, print, and social media, as well as GAF Roof Views. He joined the GAF team in late 2016, bringing a background as a creative director and author.
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