Prevent Black Streaks on Roof

Why it’s there and how to deal with it

Black streaks on shingled roofs are a common, though mysterious sight. While not immediately damaging, they can prematurely age shingles. Here’s what they are and what to do about them.

By the DIY experts of The Family Handyman Magazine

What makes the streaks?

The black streaks running down roofs are actually a hardy algae called Gloeocapsa magma. As the blue-green algae accumulate, they develop a dark, hardened outer coating, which results in the black stains you see. The algae feed on the limestone in shingles.

The algae will worsen and become more noticeable each year, trapping moisture and causing premature shingle aging and granule loss. If you don't like the streaks, reshingle with algae-resistant shingles. If the streaks don't bother you or you're not ready to invest in a new roof, install zinc or copper strips (available at roofing centers) along the top course of shingles. When rain hits the strips, it produces a solution that runs down the roof and keeps algae, moss and fungus from growing. However, the strips won't eliminate the existing algae.

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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Hammer
  • Pry bar

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

  • Zinc or copper strips