The black mold-like stains and streaks that appear on roofs, particularly light-colored asphalt shingles, are actually a blue-green algae (Gloeocapsa magma). Commonly found in climates with warm, humid summers – like Baltimore – it does no damage to the roofing, even though it may be unsightly.
Sometimes mistakenly called “mildew” or “fungus”, algae staining on asphalt shingles usually appears in shaded areas or on the more-shaded roof slopes – specifically in areas of the roof under trees.
Algae is common on roof surfaces of several materials including asphalt shingles, concrete and clay roof tiles, and even slate.
Its growth triggered by dampness, algae is a living organism that thrives on dust, pollen, and the organic materials that make up the roof. It grows on, under, and behind the tiny granules that cover the shingles and can harbor a number of molds. This results in stains and streaks on the roof – again, unsightly but not damaging.
If you are considering buying a home, and have some concerns about stains that you see on the roof, notice their placement. Are they under a large tree, or on a side of the house that stays mostly shaded? Do the stains cover the entire roof, or only appear in patches? Most likely this is algae, which does not damage the roof. Ask the listing agent for more information.
Algae on asphalt shingles is a cosmetic or aesthetic issue, not a roof performance (durability) problem, and it is generally not necessary to remove and replace asphalt shingles strictly because of algae growth and the associated discoloration.
If you want to get rid of the algae stains on a perfectly good roof, This Old House recommends the following:
Spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae. (No pressure washers, please. They’re likely to damage the shingles.) Just be sure to wet your foundation plantings first, and rinse everything in clean water when you’re done. Plants don’t like bleach, and wetting them with plain water first protects them.
To keep the algae from coming back, insert 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. That way whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles.