Pressure Washing
Those dark, dripping streaks running down a roof are not dirt, soot, or age — they are a living blue-green algae called Gloeocapsa magma that feeds on the limestone filler in asphalt shingles. Rain spreads the spores downward, which is why the stains streak.
The instinct is to climb up with a pressure washer and blast the streaks off. Don't. High pressure strips the protective granules off shingles, voids many manufacturer warranties, and can force water under the courses. It also only removes the surface layer — the algae grows right back within months.
A proper roof cleaning uses soft washing: low pressure paired with a cleaning solution that kills the algae at the root. Done correctly, the roof rinses clean and stays clean far longer because the organism itself is dead, not just knocked loose. Most reputable crews follow the same low-pressure standard the asphalt-roofing industry recommends.
North-facing and shaded slopes stain first because they stay damp longest. If your roof re-streaks quickly, a strip of zinc or copper flashing installed near the ridge releases trace metal ions when it rains, which suppresses regrowth. In hot, humid regions this problem is relentless — homeowners in the Upstate deal with it constantly, and the crew we point Greenville folks to handles roofs on low pressure as a rule. Ask any pro to confirm they soft-wash roofs before you book.