Those dark streaks on many asphalt-shingle roofs are often associated with an airborne blue-green algae commonly called Gloeocapsa magma. The staining can become more visible as colonies spread across shaded or moisture-prone roof areas.
Why streaks appear
Moisture, shade, roof orientation, nearby trees, and materials within the roofing system can affect growth. The streaks are not simply ordinary dirt, and aggressive cleaning can damage shingles.
Do not pressure-wash a roof
Walking on a roof and using high pressure can create safety risks and shorten the life of roofing materials. Cleaning should follow the manufacturer’s instructions and account for landscaping and runoff.
Think about long-term ownership
If an aging asphalt roof is already near replacement, repeated cosmetic treatment may not address the larger issue. Permanent aluminum roofing will not rust and provides a very different exterior surface.