What are Those Black Streaks on My Roof?

The black streaks on your roof didn’t come from being dirty.

Black streaks running down your roof are a type of roof algae knowns as Gloeocapsa Magma. These pesky bacteria tend to cluster in groups and locate themselves typically on the north side of roofs where the lack of sun can stimulate their growth. They like to feed on limestone - a key filler ingredient in asphalt shingles.

For the ones that are exposed to sunshine, they have a unique method of survival. According to the microbe wiki at Kenyon University, when exposed to strong sunlight, the bacteria forms a black/dark green sheath around itself as a form of protection from ultra-violet rays. The pigmented covering is what shows up as dark streaks on the roof.

You may have noticed that the stains don’t appear below any copper- or zinc-coated metal flashings that you may have on the roof around chimneys or vents. This is because the coating is toxic to the algae. Each time it rains, trace amounts of the metal are washed down the roof, preventing the growth in that area.

The bad news is that if your roof is plagued by algae, it’s not going to go away. Cleaning will remove the stains but they usually return. Repeated cleaning can actually cause damage to the shingles if harsh chemicals and power washing is used.

The good news is that shingle manufacturers have begun making shingles that are resistant to these algae. We offer CertainTeed Landmark® shingles that feature a ten-year StreakFighter® algae resistance warranty.  Since studies show that your roof can account for up to 40 percent of your home’s curb appeal, when it’s time to reroof, consider choosing an algae-resistant shingle.

At Texas Traditions Roofing, we can advise you about options and systems that can meet your home’s needs. Schedule a free assessment today.