In 2009, I wrote about the development of algae-inhibiting surfacing granules for asphalt shingles and the then-available algae-resistant asphalt shingle products and associated warranty terms (see "The fight against algae," April 2009 issue). But the industry has evolved since then, and there have been some changes in the current algae-resistant shingle market and associated product warranty terms.
The dark stains noticeable on rooftops in many parts of the U.S. and Canada are accumulations of blue-green algae, single-cell organisms typically from the family Gloeocapsa. The stains are readily recognizable by their resemblance to black ink blots and appear darkest near the top of a roof and may thin out as they extend downslope on a roof's surface. Some severely affected roofs have dark stains from eave to peak.
Roofs are not the only locations where blue-green algae thrive in urban environments. Similar staining commonly is visible on stucco and limestone walls, limestone and marble columns, and staircases. There is no evidence blue-green algae harms asphalt shingles, and asphalt shingle manufacturers consider it an aesthetic issue.