Roof CleaningIn Oakland County, MI

The dark streaks running down so many Michigan roofs are not dirt or mildew. They are a blue green algae called Gloeocapsa magma. Its spores travel on the wind and settle on shingles, where the algae feeds on the limestone filler that manufacturers mix into asphalt shingles.

As a colony grows it builds a dark outer coating to protect itself from the sun, which is why the streaks look black. It spreads fastest on north and east facing slopes that stay shaded and damp, and under trees that keep the roof from drying. Moss and lichen often follow in the same areas, and they hold water against the shingles and can lift their edges.

Asphalt shingles are covered in mineral granules that shield them from UV. A pressure washer strips those granules, which ages the roof and can cause leaks. The Asphalt Roofing Manufacturers Association advises against pressure washing shingles for exactly this reason.

A soft wash uses chemistry instead of force. The industry standard solution is diluted sodium hypochlorite, the active ingredient in household bleach, mixed with a surfactant so it clings to the shingles long enough to work. It kills the algae, moss and lichen on contact, and it is applied at about the pressure of a garden hose, so nothing is blasted off the roof.