What Are Those Ugly Black Streaks on My Roof?

Those dark streaks aren't dirt. They're algae, and while mostly cosmetic, they can shorten your roof's life over time.

Those unsightly black or green streaks you see on many roofs in our humid climate are a type of algae called Gloeocapsa magma. The algae feeds on the limestone filler in older asphalt shingles. While it doesn't cause immediate structural damage, it is a sign of aging, and by holding moisture, it can accelerate the deterioration of the shingles. Modern shingles, like those from IKO and Owens Corning, are manufactured with copper granules that are algae-resistant, preventing these streaks from forming and keeping your roof looking beautiful for years.