The black roof stains, often mistaken for “roof mold or acid rain stains” are actually the dead cells of hardy algae called Gloeocapsa Magma. The algae will usually show up on the north side of the roof first as that side 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.
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.
Roof moss and lichens anchor into the top layer of the shingles and remove granules. Moss and lichens grow up into the gullets (slots) of the shingles and along the edges, causing them to lift and make the roof more susceptible to wind damage and leaks. Acting like sponges, moss and lichens keep the roof from drying out; this perpetual dampness causes even more growth.
Cleaning roof shingles with pressure washing often dislodges the protective granules from the surface of the shingles and scars the shingles. It does not solve the root cause of the stains – algae growing and feeding on the limestone filler in the shingle. Pressure washing should not be attempted because it can reduce the life expectancy of your roof and can void your warranty.
We here at Roof Cleaner only use the shingle manufactures recommended way of cleaning shingles. We DO NOT use pressure washers to BLAST away the stains.