Why Does My Roof Have Black Streaks?

Those black streaks on your roof look familiar.  You’ve seen them on roofs before, but what are they?  Signs of aging?  Deterioration?  Both?  Actually, the answer is neither.

What Are Those Black Streaks?

Those black streaks are actually blue-green algae, formally known by its scientific name: Gloeocapsa Magma. The algae thrive in humidity, so unlucky news for you if you live in a steamy area of the United States.  The algae spread by means of airborne spores.

Black streaks are often confused with shingles-gone-bad, years of grime and dirt build-up, or even mold or mildew.  In the United States, you’ll spot the blue-green algae and the noticeably black streaks staining asphalt roofs largely in the Midwest, East Coast, and along the West Coast.  In the Western United States, where the humidity isn’t so much a factor and homeowners are instead faced with dry heat, there are low levels of algae growth.

If you look even closer, you’re bound to notice that the black streaks are everywhere but one place in particular: near the metal flashing that’s around chimneys and roof vents.  The algae have a weakness and it’s the copper and zinc coating used on the sheet of metal flashing, which is toxic and kills the growth.

When it rains, the water washes down traces of these toxic metals in trails down the slope of the roof.  The algae are killed in its wake, thus leaving behind an array of black streaks.

Do Black Streaks Hurt My Roof?

The black streaks look worse than they actually are.  The algae aren’t detrimental to your roof’s integrity, but the growth and black streaks are an eyesore.  The worst you have to worry about is your curb appeal.  If you’re selling your home, you’ll want to tackle the algae to boost the home’s value.  Even though you know it’s not the shingles deteriorating, potential homebuyers might not.

What Do I Do About the Black Streaks?

Now that you know that you’re dealing with algae and not shingle deterioration, you can do something about the unsightly black streaks.  There are ways you can clean off the algae yourself, although this isn’t advisable.  You’ll absolutely want a professional to do the job, and here’s why:

Harmful Chemicals

Killing the algae that are causing the black streaks requires some heavy-hitting chemicals.  You can use chemicals like oxygen bleach, which is less harmful to you and the environment than chlorine bleach, but it may not be as effective.

Many products are designed to kill this specific type of algae-like Wet & Forget Outdoor, or a mixture of trisodium phosphate (TSP), bleach, and water. However, most of the time, the streaks won’t come off in one application. A professional is much better equipped to handle these harsh chemicals, their mixtures, and their application to remove the black streaks.

Damage to Your Roof

Old roof with hail damage, chalk circles mark the damage. Shallow depth of field

To truly remove the black streaks from your roof, a pressure washer might be required to do the job.  Power washing your roof must be done in a certain way.  Otherwise, you’ll damage the shingles or risk exposing the roof to excessive moisture.

Both will shorten the lifespan of your roof.  It’s best to have a professional roof cleaner pressure wash the black streaks as they’re trained to use the equipment in a way that won’t damage your roof.


Cleaning your roof yourself is a dangerous task, and when it’s wet, it’s slippery.  The algae growth also makes the roof surface slick.  You’re also working with harsh chemicals, and protective gear is needed to handle them.

Undertaking this task on your own demands the appropriate safety gear like a harness, rope, gloves, goggles, and slip-resistant boots.  Consider having a professional roof cleaner tackle the cleaning your roof instead.  They’re practiced in roof safety, fully equipped for slippery surfaces, and have the protective gear ready for the harsh chemicals.

How Do I Prevent Black Streaks on My Roof?

Algae Resistant Shingles

Shingle manufacturers have gotten wise to the problem of algae that plagues a good portion of roofs across the United States.  Now, there are algae-resistant shingles.  If you’re in need of a roof replacement and have noticed that black streaks are a persistent problem then consider asking your roofing contractor about this type of shingle.

The shingles are manufactured with a mix of copper granules, which as you’ll recall, the algae despise. If you can prevent black streaks and algae from forming on your roof, you can save yourself the expense of having to clean it.

Metal Strips

If a roof replacement or upgraded algae-resistant shingles aren’t in your future anytime soon, there’s another prevention trick you can try.  Install a strip of copper or zinc-coated sheet metal along the side of the roof, just beneath the ridge—about 2”-4” of metal should be seen on the roof for it to be truly effective in killing algae growth.

Copper is the best in fighting algae because it’s more toxic, but galvanized sheet metal is more cost-effective and will also work.

Asphalt Shingles and Professional Installation

Widely popular across the Midwest, asphalt shingles are made to withstand strong winds, rain, and snow. On top of that, asphalt shingles have varieties of colors and styles that will significantly boost your curb appeal. However, not all companies have a strong reputation for installing asphalt shingles correctly.

For instance, your shingles can warp and lift up if the sealant strip is not properly installed. That’s why many homeowners choose Feldco for roof replacement. Feldco has a great reputation for great products, services, and professional installation. Speak to a product specialist and get a free quote online today.

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