While you might think it is built-up dirt, a black stain on roof shingles is caused by algae. You are probably thinking to yourself, “Isn’t algae green?” The most common kind of algae that grows on roofing is actually a blue-green hue; however, this specific type, which is called gloeocapsa magma, changes in color. It creates a dark coating, which appears as black, to protect itself from the sun’s UV rays. Mass amounts of the algae grouped together create the appearance of the dark, black streaks that most homeowners believe to be dirt or soot.
How does the algae get on the roof?
Most likely the algae was brought to your roof by wind or animals. The algae spores are airborne. So, if homes in your surrounding area have black stains, there is a likelihood that the algae traveled to your home from those roofs.
Why does the algae seem to make such large stains?
The algae may start as a single spot, but if left untreated, it can grow into large masses. It feeds off of the shingles. Although you may have what is referred to as asphalt shingles, the shingle material is only comprised of asphalt. It also contains fillers like crushed limestone, which the algae feeds off of. The more the algae eats, the more it grows and spreads, creating those long black stains that look like streaks.
Why are the black stains only on one side of my house?
The amount of sun and shade the roof gets during the day plays a big part in where the algae grows. Chances are, the part of your roof with the most stains faces the north. This area of the roof is in the shade for most of the day, which results in more moisture, encouraging algae growth.
Do I need to clean the algae off of my roof?
Yes! The algae are slowly deteriorating your shingles. Over time, this can lead to water issues.