Maintenance · 4 min read
Black Streaks on Your Roof: What They Are and How to Remove Them
Published
Those dark streaks running down north-facing slopes aren't dirt and they aren't damage — but they do tell you something about your roof's environment.
What it is
Gloeocapsa magma, a blue-green algae that feeds on the limestone filler in asphalt shingles. It thrives in humidity and shade, which is why north slopes and tree-covered homes in West Milton and Englewood get hit hardest.
Does it harm the roof?
Minimally in the short term, though heavy growth retains moisture and can accelerate granule loss over years. Moss is a different story — it lifts shingle edges and holds water directly against the mat.
Safe removal
A 50/50 mix of household bleach and water applied with a pump sprayer, left to work, then rinsed gently. Never pressure wash a shingle roof — it strips granules and takes years off the roof for a cosmetic improvement.
Prevention
Algae-resistant shingles with copper-infused granules cost slightly more and stay clean. A zinc or copper strip installed near the ridge also works: rain carries trace metal down the slope and inhibits growth.
If any of this applies to your own home, our asphalt shingle roofing crews handle it every week, and you can see how we work in your area on our Troy, Ohio roofing page. Inspections and written estimates are free either way — call 937-749-7767.