Warm, humid and shaded: ideal for growth
The black streaks on north-facing roof slopes are a colony of cyanobacteria, a tiny organism related to algae, feeding on the limestone filler in shingles. It is alive, it is common across this region, and pressure does not remove it. The right treatment is a cleaning solution applied at low pressure.
On siding, the same conditions grow mildew and algae on shaded walls. High pressure strips the growth and drives water behind the siding at the same time, which is why we soft wash walls.
Concrete and masonry are different. They can take real pressure, and driveways, patios and walkways are where a pressure washer belongs.














