Black streaking on a Menands 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.
Between the Hudson right below and the traffic corridors running through it, Menands rooflines pick up both organic growth and a fine road-grime film that dulls shingles years before they wear out.
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.