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Dark Streaks on Your Roof? Here’s Why

If you have a light-colored roof, you may have noticed dark or even black streaks developing on the shingles and wondered whether this is dirt, moss, or fungus and if the accumulation is causing damage to your roof. Most often, these black streaks are caused by a blue-green algae and typically, the growth will not cause damage to your roof.

Algae are plant-like bacteria that easily spread from rooftop to rooftop because their spores are carried through the air. Algae is most likely to grow in hot, humid climates (similar to Midwest summers) and feeds on the limestone granules that are added to many shingles. The most common type of algae that grows on roofs is called gloeocapsa magma. To protect itself from the harmful UV rays that bare down on it, the algae produces a dark pigmented coat. This pigmentation is what causes you to notice the dark discoloration on your roofs.

Typically, algae growth does not harm the roof, but very extensive growth can cause damage if algae and fungus grow together to form lichen which further feeds on the granules of the shingles.

To prevent algae growth, many shingle manufactures are now adding copper or zinc granules to the shingles. These copper or zinc granules are lethal to algae and can prevent their growth. Nevertheless, in especially hot and humid climates, or on roofs without algae-resistant shingles, algae is likely to accumulate and become noticeable on light-colored roofs. If the streaks bother you, the algae should be dealt with appropriately by a roofing professional. Using a pressure washer, excessive scrubbing, or applying harsh chemicals will damage the surface of the shingle, which not only makes your roof more susceptible to leaks, but may also void the manufacturer’s warranty.

To keep algae growth on your roof under control, the following recommendations should be followed:

  • Trim tree limbs that hang over the roof to let sunlight reach the roof
  • Regularly clean your gutters (read how to here) so that water can drain freely from the roof
  • Regularly remove debris from your roof using a leaf blower
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