Curious About The Black Streaks On Your Roof? Here’s Scoop!

Curious about the black streaks on your roof? Here’s the scoop!


The dark black/brown streaks that you see on your roof is an Algae called Gloeocapsa magma. Depending on the age and condition of your residential roof, cleaning may be the most cost-effective solution, since it’s about 5 to 10 percent of the price of a roof replacement. Algae-caused marking isn’t preventable but can be removed, though not always permanently. The algae survive through photosynthesis and by feeding on limestone filler used in asphalt shingles. Black marks became a problem about 20 years ago when manufacturers began adding limestone granules to add weight to roof material used to coat shingles.

Most black streaks form on the northern slopes of roofs, where it is darker and wetter – ideal for algae growth. The algae appear blue-green when the organisms form an outer coating to protect themselves from ultraviolet rays. When decayed, the algae turn black. Algae and fungus can grow together to form lichen, the roots of which can wrap around and feed on the granules covering the shingles. Once this happens, lichen is not easily removed. Even if it dries out, it can come back to life with the next rain. Scrubbing or power washing will only cause more damage. Today’s Homeowner mentions in this blog post that algae stains are absent directly below metal flashing around chimneys and/or roof vents because the copper and zinc coating on galvanized sheet metal are toxic to algae. Everytime it rains, trace amounts of metals are washed down the roof, inhibiting algae growth. In recent years, roofing manufacturers have begun mixing copper granules into roofing products to produce algae resistant shingles. 

If you suspect that your roof is damaged in this manner or another, give us a call to set up a free 30 point roof inspection to receive a clear diagnosis. We can be reached at 704.663.7521 or

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