What Are the Black Streaks on My Roof?

If you've noticed dark black streaks running down your roof, you're not alone. This is one of the most common issues for Minnesota homeowners, and it's not dirt or mold — it's a blue-green algae called Gloeocapsa magma.

This algae feeds on the limestone filler used in asphalt shingles. It thrives in humid environments, which is why it's so common in Minnesota's summer months. North-facing roof sections and areas shaded by trees are hit hardest because they retain moisture longer.

The algae doesn't just look bad — it actually damages your shingles over time. As it feeds on limestone, it loosens the granule layer that protects shingles from UV damage. Left untreated, it can shorten your roof's lifespan by several years.

Many Plymouth homeowners assume they need a new roof when they see these streaks. In most cases, a professional soft wash can remove the algae completely and restore the roof's appearance for a fraction of the replacement cost.

Important: never let anyone use a high-pressure washer on your roof. The force will strip granules, crack shingles, and void most manufacturer warranties. Soft washing uses low pressure with a bleach-based solution that kills the algae without causing damage.

Prevention: some newer shingles contain copper granules that resist algae growth. Zinc strips installed along the ridge can also help. But for existing algae, soft washing every 2-3 years is the most effective maintenance approach.