Why black streaks appear on Michigan roofs — and how to remove them without damage

You've seen it on half the roofs up here: dark streaks running down the north- and shade-facing slopes. It looks like dirt or water staining. It's neither.

It's algae, and it's eating your roof

The streaks are Gloeocapsa magma, a hardy blue-green algae. It feeds on the limestone filler baked into asphalt shingles, spreads across the roof, and holds moisture against the surface — which shortens the shingle's life. It shows up first on the slopes that stay damp and shaded, exactly what northern Michigan's tree cover and humidity provide.

Why you can't just power-wash it off

Pressure-washing a roof strips the granules that protect the shingle from UV, knocks years off its life, and voids most shingle warranties. It also doesn't kill the algae — so it comes right back.

The safe fix: soft washing

A soft-wash treatment applies a solution that kills the algae and mold at the root with no high pressure at all. The staining lightens immediately and continues to clear as it weathers off over the following weeks. The roof — and the warranty — stay intact.

How often up here

Most northern-Michigan roofs go several years between treatments, sooner on heavily shaded north slopes near the water. If your neighbor's roof is streaking, yours is next.