That black streak on your roof is actually alive

Those dark streaks running down a Florida roof are a hardy blue-green algae called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles, which is why the staining spreads and comes back if you only rinse it off.

Blasting it with a pressure washer is the worst thing you can do — it strips the protective granules off the shingles (often voiding the warranty) and doesn't kill the organism, so it grows right back. The correct fix is a low-pressure soft wash with the right solution that kills the algae at the root.