by Angie Hicks, Founder of Angie’s List
You may have the most attractive landscaping, beautiful windows and charming mailbox, but black streaks running down your roof can ruin your home’s curb appeal.
What causes those dark marks, and what can be done to eradicate them?
Roof experts who are highly rated by members of Angie’s List say the source is an algae called Gloeocapsa magma. And depending on the age and condition of your roof, cleaning may be the most cost-effective solution, since it’s about 5 to 10 percent of the price of a roof replacement, which can be as much as $10,000.
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 material used to coat shingles.
Roof experts tell our researchers that though other components are being added to shingles to hinder algae growth, they still get calls to deal with black marks on relatively new roofs. They say that while shingle manufacturers offer products treated with copper or zinc to inhibit algae growth, their effect wears down over time.