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.