Black streaking on a Bethlehem 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.
Bethlehem's oaks and maples drop debris into valleys all fall, and the shaded slopes through Elsmere, Selkirk, and Cedar Hill hold enough moisture to grow moss on top of the usual black streaking.
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.