Shingle under the street trees
A city planned around trees and landscaped roads puts a lot of roofs in partial shade, and the townhome and villa rows from the 1980s and 1990s are where shingle is most common. Shaded shingle grows Gloeocapsa quickly, and the black streaks run down from the ridge in stripes that follow the rain.
Pressure washing shingle strips the granules that shield the asphalt from the sun, and once they are gone the shingle ages fast. The soft wash kills the algae and lichen at low pressure and leaves the granules where they are. The dead growth keeps lightening with the next few rains after the clean. Gutters and downspouts are flushed at the end so the runoff does not sit in them.


