The answer to 'why is my asphalt shingles algae growth' is that algae growth on asphalt shingles is a common problem caused by the moist, shaded conditions that allow algae to thrive. Algae feed on the limestone filler in asphalt shingles and can cause unsightly black or green streaks on the roof.
Algae growth is more likely to occur in humid climates, on north-facing roofs, or in areas with a lot of shade from trees or other structures. Over time, the algae can degrade the shingles and lead to leaks and other roof damage if left untreated.