Black streaking on a Ravena roof is Gloeocapsa magma, a hardy algae that feeds on the limestone filler in asphalt shingles. It starts on the shaded slope and spreads across the whole roof over a few seasons.
Village roofs in Ravena see plant and road dust along with the standard Capital Region algae, and the older, lower-pitch roofs through the village center hold that debris instead of shedding it.
Pressure washing a roof strips granules and shortens its life. We use the low-pressure soft wash the shingle manufacturers actually recommend: apply, dwell, rinse gently. Streaks are gone the same day and stay gone for years.