"Why does my neighbor's roof look fine, while mine is completely black?"

1. Humidity Levels

Gloeocapsa magma thrives in high humidity. Regions in the Southeast US experience the worst streaking because the constant moisture in the air prevents the roof from fully drying out.

2. Shade and Tree Cover

Direct sunlight kills algae by drying it out. Large overhanging trees that cast deep shadows on the roof create a cool, damp microclimate where algae spores can multiply rapidly without UV interference.

3. Roof Orientation (North Slopes)

In the Northern Hemisphere, the north and east-facing slopes of a roof receive the least direct, intense sunlight. These slopes are almost always the first to show severe black streaking.

4. Proximity to Water

Homes located near lakes, large ponds, or the ocean naturally experience heavier morning dew and fog, providing the daily hydration the algae colony requires to survive.

5. Shingle Composition

Older roofs made before the widespread use of limestone filler rarely had algae problems. Modern fiberglass-asphalt shingles packed with calcium carbonate act as an all-you-can-eat buffet for the cyanobacteria.

6. Roof Pitch (Steepness)

Lower pitched (flatter) roofs drain water more slowly than steep roofs. Slower drainage means the shingles stay wet longer, increasing the window of time algae has to absorb moisture.

7. Prevailing Winds

Algae spreads via airborne spores. If your home is downwind from a heavily infested neighborhood, your roof will be constantly bombarded with new spores looking for a home.

8. Roof Age

Even if you have Algae Resistant (AR) shingles, the protective copper granules eventually wear off. Once a roof ages past its algae-resistance warranty period (commonly 10 to 25 years, depending on the manufacturer), its natural defenses are depleted.

9. Previous Treatment History

If a roof is cleaned poorly (e.g., using weak hardware store chemicals instead of professional soft washing), the root of the algae survives and the black streaks often return quickly, faster and thicker than before.