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Prevent Black Streaks on Roof

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

Comments from DIY Community Members

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March 28, 8:30 AM [GMT -5]

I've always wondered about this. Thanks Family Handyman!

September 09, 7:18 PM [GMT -5]

Sorry, meant to say QSE 3000. Just a typo

September 09, 7:15 PM [GMT -5]

Inquired a friend that had the same thing and said he used QSO3000 from cleanertoday dot com. I bought the kit for about 100 bucks and it worked like a charm. At first it didn't seem to have cleaned it enough, but after a few rain falls, looks like new

September 09, 5:54 PM [GMT -5]

The streaks become more noticeable depending on pollution in the area. I would NEVER hire anyone to clean my roof. The use of bleach [toxic to vegetation around home] will rapidly age the shingles, also it's applied with a pressure washer. A pressure washer will remove the gravel on the shingles & take years off the shingles. Dark shingles don't show the streaking neither does a tin roof.

August 05, 3:30 PM [GMT -5]

The zinc and copper stripes works to an extent. The problem with this is that it only works the first 5 - 7 feet generally. You could string copper wire in intervals across the roof.

There are also many professional roof cleaning companies that use a bleach to water solution to clean black streaks from roofs. Hope this helps anyone who has questions regarding this. Garret

July 20, 3:15 PM [GMT -5]

I've never heard of this before, but love this idea.
We have a cistern in which the water supplies our showers, toilets and washing machine. Is this a still safe "black streak" preventative?

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