What are those black streaks running down my roof?

WHAT are they?

The black streaks you see are gloeocapsa magma algae. As they mature, these blue-green algae harden and darken, becoming more visible and more damaging to your shingles over time.

WHY is it growing on my roof?

The algae are actually feeding on your roof. Composition shingles contain various fillers — one of the most abundant is limestone, chosen for its cost-effectiveness and availability. While the limestone fights back against the algae, it will never fully win. As the algae hardens, the limestone inside the shingles is gradually consumed and weakened.

DO I need a new roof?

If your roof has gloeocapsa magma growth, you’re not alone. Most roofs over 10 years old will start showing signs of algae, and it will continue to spread. You might think your roof still has 5, 10, or even 15 years left — but the algae can significantly reduce its lifespan by slowly eating away at the shingles.

If the streaking is widespread or if you’re planning to sell your home, replacing the roof might be wise. Extensive algae growth indicates long-term shingle degradation, weakening your roof’s structure. Plus, prominent black streaks can become a bargaining chip for potential buyers, who may push for price reductions. Realtors often advise clients to check the roof first since it’s one of the most significant expenses a homeowner can face.

HOW can I keep the black streaks away?

• Consider installing algae-resistant shingles when you replace your roof.
• Install copper or zinc strips along your roof to help prevent algae growth.
• Use a mixture of TSP, bleach, and water to temporarily remove staining — but avoid excessive scrubbing to prevent shingle damage.