Those dark streaks running down asphalt shingles are most commonly caused by a type of blue-green algae known as *Gloeocapsa magma*. The algae can spread through airborne spores and tends to thrive in warm, humid areas where moisture lingers on the roof.

As the algae grows, it develops a dark protective coating, creating the black or charcoal-colored streaks you see running down the shingles. Because moisture encourages its growth, staining is often more noticeable on shaded areas of a roof—especially north-facing slopes or sections beneath trees that receive less direct sunlight.

Asphalt shingles can provide another ingredient the algae needs. Many shingles contain limestone filler, which can provide nutrients that help the algae continue to grow and spread.

That’s why the streaks often become more noticeable over time instead of simply washing away with the next Wisconsin rainstorm.

Do Black Streaks Damage Your Roof?

Black streaks can make a roof look much older than it really is, but seeing them doesn’t necessarily mean your shingles are damaged or that your roof needs to be replaced.

In many cases, the biggest immediate concern is appearance. However, algae growth can indicate that portions of the roof are staying damp for extended periods—and moisture, shade and organic growth can create conditions that aren't ideal for your roofing system.

It’s also important to distinguish algae staining from other roof problems. Moss, lichen, missing granules, curling shingles and deteriorated roofing materials can sometimes appear alongside dark staining but may require very different attention.

That’s why the overall condition and age of the roof matter more than the color of the streaks alone. If you're unsure whether you're looking at harmless staining or signs of deterioration, a professional roof inspection can help determine what’s actually happening.

Bottom line: Black streaks aren't automatically a sign that you need a new roof—but they're worth taking a closer look at, especially if your roof is older or showing other signs of wear.

Why You Should Never Pressure Wash Asphalt Shingles

A pressure washer might make quick work of dirt on a driveway or deck, but it can be far too aggressive for an asphalt-shingle roof.

Asphalt shingles are covered with tiny mineral granules that help protect the underlying asphalt from sunlight, weather and everyday exposure. High-pressure water can loosen and strip away those granules, leaving the shingles more vulnerable to deterioration.

Pressure washing can also force water beneath shingles and into areas where it doesn't belong. Depending on the roof's condition and how the water is directed, that moisture can potentially reach the roof decking or other components beneath the shingles.

There's another problem: blasting away the visible staining doesn't necessarily address the algae itself. If the underlying growth isn't properly treated, those dark streaks may eventually return.

And there's the safety issue. Walking on a wet, sloped roof while handling pressure-washing equipment can be extremely dangerous.

The better approach: If black streaks are bothering you, have the roof evaluated and use a cleaning method appropriate for asphalt shingles. A gentler treatment can address algae growth without subjecting the roofing system to unnecessary pressure.

Your roof doesn't need a power wash—it needs the right treatment.

How Algae-Resistant Shingles Help Prevent Black Streaks

If you're replacing your roof, you don't necessarily have to accept black streaks as something that will eventually come with it. Many modern asphalt shingles are designed with algae-fighting technology that helps prevent the growth responsible for those dark stains.

Algae-resistant shingles typically incorporate specially engineered granules containing copper or other algae-inhibiting materials. As the roof is exposed to moisture over time, these components help create an environment that makes it more difficult for algae to grow and spread across the shingles.

For Wisconsin homeowners, that can be especially valuable. Our combination of summer humidity, rainfall, shade and tree cover can create ideal conditions for roof algae.

Badgerland installs GAF roofing systems that offer algae-protection options designed to help roofs maintain their appearance longer. Certain GAF shingles feature StainGuard Plus™ Algae Protection Limited Warranty coverage, providing additional protection against the blue-green algae that causes those familiar black streaks.

Of course, algae resistance doesn't mean a roof will never get dirty or require maintenance. But choosing shingles designed specifically to resist algae growth can make a significant difference in how your roof looks as it ages.

If you're already planning a roof replacement, algae resistance is one feature that's worth considering—especially if black streaks are common on homes in your neighborhood.