What Are Those Black Stains on My Roof?
These black roof stains, often mistaken for “roof mold,” are the dead cells of hardy algae called Gloeocapsa Magma. The algae grow on the north side of the roof that gets the least amount of sun and stays damp the longest. They feed on limestone filler in the shingles and grow and darken until removed. Airborne algae spores are then carried from roof to roof and can gradually infest an entire neighborhood.
These algae appear black because the organism Gloeocapsa produces a dark-pigmented covering to protect itself from the ultraviolet rays of the sun. As the algae on your roof shingles die off, the dead cells accumulate and cause the black streaks sometimes referred to as “roof mold.” These unsightly stains take months, or even years, to appear.
The growth of algae can cause damage and severely reduce your roof’s useful life if left unattended. Algae feed on the limestone filler used to give the shingles weight. Over time, the ceramic granules protecting the surface of the shingle begin to loosen and wash off with the rain. This reduces the roof’s ability to reflect heat rays, which can result in the roof overheating, thereby increasing air conditioning bills and shortening shingle life. Algae presence on roof shingles also enables other damaging biological growth, such as lichens and roof moss.