Dark roof streaks are commonly associated with algae growth that feeds on components in asphalt shingles. Moss, lichen, tree debris, shade, moisture, and poor airflow can create additional staining or deterioration. The appearance alone does not always tell you the roof's structural condition.

Aggressive pressure washing can damage shingles and shorten roof life. Cleaning methods should follow the roofing manufacturer's guidance and use appropriate low-pressure application, plant protection, runoff control, and safe roof-access practices.

Before cleaning, inspect for loose shingles, active leaks, damaged flashing, excessive granule loss, or areas that may require a roofer rather than a cleaning service.