What causes algae on roof shingles?

Algae on roof shingles is caused primarily by a specific type of cyanobacteria called Gloeocapsa magma, which thrives in warm, humid environments and feeds on the limestone filler used in asphalt shingles. This bacteria is airborne and spreads via spores carried by wind, animals, and insects, so any roof can become infected, especially in humid climates. Several environmental factors promote algae growth. Shade is a major factor, as north-facing slopes and areas shaded by trees stay damp longer and receive less UV radiation that would otherwise kill algae. Moisture retention from poor drainage, clogged gutters, and debris buildup creates a damp environment where algae flourishes. Poor attic ventilation can also contribute by causing condensation and keeping shingles damp. The limestone filler in modern asphalt shingles, added to add weight and reduce manufacturing costs, serves as a food source for algae, which is why algae streaks have become more common since limestone became standard in shingles. Once established, algae forms a dark pigmented coating that appears as black streaks running down the roof, as rain washes the bacteria downward. While algae does not immediately destroy shingles like moss does, it retains moisture and can accelerate granule loss over time, and it significantly affects curb appeal.