What Are Those Dark Streaks On My Roof?

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Algae Growth on Asphalt Roofing Shingles

If you live in a humid area of the country, you’ve probably seen unsightly dark streaks on asphalt shingle roofs. Like many, you may have thought that was the result of dirt or mildew buildup, defective shingles, or tree droppings; however, the most common culprit is actually a blue-green algae known as Gloeocapsa Magma who’s growth spores happen to travel by air.

Can You Remove Algae Discoloration, from an Asphalt Roof?

Algae discoloration can be difficult to remove, but may be lightened by spraying a diluted solution of chlorine bleach and water on the roof.

Disclaimer: When deciding to clean your roof we recommend you consult a licensed/certified roofing professional. If you decide to clean your roof yourself, please keep in mind that the roof will be slippery when wet and caution should be taken when performing this task. Wear appropriate fall protection when on the roof and proper PPE, such as slip-resistant shoes/boots and eye protection while cleaning the roof.

How to Lighten Dark Streaks, or Algae Discoloration, on an Asphalt Roof

  • Prior to applying the solution, all ground vegetation directly under the roof should be covered.
  • The solution should be a mixture of one gallon each of bleach and water (1:1).
  • This solution should be sprayed on the roof and left untouched for approximately 10 – 20 minutes.
  • Do not scrub or use high-pressure power washing equipment. This will loosen and remove granules, thereby shortening the life of the roof covering.
  • After approximately 10 – 20 minutes, the roof should be sprayed with water, taking care to thoroughly flush the roof and the greenery around the home (grass, bushes, and shrubs).
  • When algae growth is extensive the cleaning process may need to be repeated. This technique is typically temporary, and the discoloration may recur.
  • If cleaned properly, you can expect your roof to stay clean for up to two to five years.

How Does Algae Affect the Performance of Asphalt Shingles?

We have not seen any evidence that algae deposits affect the overall performance of asphalt shingles. Algae discoloration is more of an aesthetic issue and may only impact the look or curb appeal of your home.

What Are Algae Resistant (AR) Shingles?

Asphalt shingles with algae-resistant mineral granules remain the most effective shingle available for reducing algae growth and therefore reducing the visible streaking described above. These products incorporate metals, such as copper-lined mineral granules, and have been shown to substantially reduce algae growth. Experts realized that metals, such as copper, have algae-inhibiting properties after noticing that algae staining was reduced or did not exist beneath metal vents or flashing on a roof. We source a complete line of algae-resistant asphalt shingles* that feature StreakGuard® Protection, an algae-inhibiting technology to help reduce algae growth. It is to be noted that these products do not address green algae (typically green in appearance) or moss sometimes found on roofs with northern exposure, overhangs, or high moisture areas.

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