Streaking on an asphalt roof is most commonly caused by algae, specifically a type called Gloeocapsa magma. This cyanobacterium creates dark, often black or greenish-black, streaks as it grows on the shingles.

Here’s a breakdown of why this happens and other potential causes:

Gloeocapsa Magma Algae:

1. Appearance: This is the primary culprit for the dark streaking. While the algae itself can be blue or green, it develops a dark, protective sheath to shield itself from UV rays, making it appear black or dark green on your roof.
2. Food Source: This algae thrives on the limestone filler that is commonly used in asphalt shingles.
3. Conditions for Growth: It flourishes in warm, humid conditions and is often more prevalent on the north side of roofs, where there is less direct sunlight and moisture tends to linger.
4. Spread: Algae spores are airborne and can easily spread from one roof to another, explaining why you might see similar streaking on multiple homes in a neighborhood.

Other Biological Growths:

1. Moss: While algae usually causes streaks, moss appears as green, fuzzy patches. It typically grows in damp, shaded areas and can lift shingles, leading to potential moisture damage and leaks.
2. Lichens: These are a combination of algae and fungus. They can appear as green, gray, or orange patches and have a strong attachment to the roof surface, making them difficult to remove and potentially damaging to shingles over time.
3. Fungus/Mildew: These can also contribute to discoloration, often appearing as black streaks or fuzzy textures, especially in humid conditions and shaded areas.

Other Potential Causes of Discoloration (less common for streaking specifically):

1. Rust Stains: Red or brown streaks can occur from rusting metal components on the roof, such as flashing, vents, or even nails.
2. Asphalt Bleed-Through: In older or low-quality shingles, the asphalt can “bleed” through to the surface, causing dark discoloration.
3. Granule Loss: As shingles age, the protective granules wear off, which can change the roof’s appearance and sometimes look like fading or discoloration.
4. Tree Debris: Accumulated leaves, needles, and branches can trap moisture and break down, creating a breeding ground for mold, mildew, and algae, and leaving stains.
5. Pollution: Airborne contaminants and industrial emissions can also accumulate on the roof surface, leading to a dirty or discolored appearance.

Impact of Streaking: While algae streaking is primarily an aesthetic issue and often doesn’t immediately affect the structural integrity of your roof, long-term growth of algae, moss, and lichens can lead to issues like:

1. Granule loss on shingles, reducing their protective qualities.
2. Moisture retention, potentially leading to deterioration of the roofing materials and, in severe cases, leaks.
3. Reduced effectiveness of “cool roof” systems by darkening the surface.

If you notice streaking or other discoloration on your asphalt roof, it’s generally best to address it to maintain your home’s curb appeal and prevent potential long-term issues. Many homeowners opt for professional roof cleaning or consider installing algae-resistant shingles for future protection.