Black Streaks On Roofs! What Are They And How To Remove?
If you’ve noticed dark, ugly streaks running down your roof, you’re not alone. It’s one of the most common questions we hear from homeowners across the Milwaukee area. And the good news is that those streaks are not a sign that your roof is failing, but they are a sign that something is growing up there that you want to get rid of sooner rather than later.
The Real Cause: Gloeocapsa Magma
Those black streaks on roofs are almost always caused by a specific type of cyanobacteria called Gloeocapsa Magma. It sounds complicated, but here’s what you need to know: it’s a living organism that feeds on the limestone filler found in modern asphalt shingles. As it spreads across your roof, it leaves behind a dark, protective pigment, which is exactly what creates those streaks.
Gloeocapsa Magma thrives in warm, humid climates, and it spreads through airborne spores. In Wisconsin, and especially here in Ozaukee and Milwaukee Counties where Lake Michigan moisture keeps humidity elevated through spring and early summer, these conditions are particularly favorable for Gloeocapsa Magma to take hold. Homeowners in Cedarburg, Grafton, Mequon, and along the lake shore in communities like Bayside and Fox Point tend to see this more than inland areas. Once it’s on your roof, it doesn’t go away on its own.
It’s also contagious from roof to roof. If your neighbors have it, there’s a good chance spores have already landed on your shingles.
Is It Actually Damaging My Roof?
Yes, and this is the part that catches a lot of homeowners off guard. Gloeocapsa Magma isn’t just cosmetic. Black streaks on roofs actively eat away at the limestone granules in your shingles over time. Those granules exist to protect the asphalt layer underneath from UV exposure and weathering. When they’re degraded, your shingles age faster.
Left untreated for years, this can shorten the effective lifespan of your roof by a significant margin. Roof replacement is one of the most expensive home improvement projects there is. Cleaning your roof is a fraction of that cost.
Beyond the bacteria itself, black streaks on roofs also absorb more heat from the sun. That extra heat can raise your attic temperature, which puts additional strain on your HVAC system during summer months. It’s a small effect but a real one.
What About Moss and Lichen?
Black streaks from Gloeocapsa Magma are often the first stage of a larger problem. Moss and lichen tend to follow in areas where the bacteria has already taken hold, particularly on north-facing roof sections that stay shaded and damp.
Moss is soft and green, and it holds moisture against your shingles. Lichen is harder and more tightly bonded; it’s a combination of algae and fungus that physically embeds into the shingle surface. Lichens are the most aggressive of the three when it comes to shingle damage, and it’s also the hardest to remove. The longer you wait, the worse it gets.
If you’re curious about lichens specifically, we wrote a dedicated piece on the topic: Why Lichens Can Destroy Milwaukee Roofs.
The Right Way to Clean a Roof: Soft Washing
Here’s what you should never do: pressure wash your roof. High-pressure water will blast granules right off your shingles, which is the exact damage the bacteria was slowly causing in the first place. You’d be accelerating the problem rather than solving it.
The correct method is soft washing , a low pressure application of a specially formulated cleaning solution that kills the bacteria, moss, and lichen at the root. The solution does the work, not the pressure. This is the method recommended by ARMA (the Asphalt Roofing Manufacturers Association), and it’s the only method that actually eliminates the organism without damaging your shingles.
After a proper soft wash treatment, you’ll typically see the black streaks totally disappear by the time we leave. Moss and lichen die off immediately after the treatment and typically will come down in the near future with rain help flushing them down. In most cases, results last several years depending on your roof’s environment and tree coverage.
What About Those Zinc Strips I’ve Heard About?
Zinc strips are metal strips installed near the ridge of the roof and are sometimes marketed as a preventive measure against algae and moss. The idea is that rainwater washes zinc ions down the roof, creating an environment where the organisms can’t grow.
They do have some preventive benefit when properly installed on a clean roof. But they’re not effective at eliminating an existing problem, the protection only extends a few feet below the strip, and they’re not a substitute for cleaning. If you already have black streaks, zinc strips won’t fix them.
Will My Insurance or Home Sale Be Affected?
In some cases, yes. Some insurance carriers have started flagging heavily stained, moss-covered roofs, and black streaks on roofs during inspections, particularly if the growth suggests the roof is being neglected. We’ve cleaned many roofs from homeowners in the Milwaukee and Ozaukee County area who were asked to have a professional cleaning as a condition of maintaining coverage or renewing a policy.
Curb appeal matters in real estate as well. Black streaks on roofs are one of the first things a potential buyer or their inspector will notice. A clean roof signals that the home has been maintained, which matters for both perceived value and the inspection process.
How Do I Know If My Roof Is a Candidate for Cleaning?
Most asphalt shingle roofs are excellent candidates for soft washing. Black streaks on roofs are bacteria, as we covered above, and they respond well to a proper soft wash treatment. Our pricing is based on square footage, and you can get an accurate quote instantly through our online quoting tool above; just click the INSTANT ONLINE QUOTE button. It’s 99% accurate and rarely needs to be adjusted after the fact. No salesperson stopping out, no pressure, no hassle, just an honest number you can act on.
As for the cleaning process itself, we work from the gutter line using ladders in most cases, applying the soft wash solution without ever needing to walk the full surface. On steeper or more complex roofs, we’ll walk it when needed. We have the equipment to handle any pitch safely. No peak is out of reach.
The one situation where we’ll pump the brakes: if your shingles are at the end of their life and replacement makes more sense than cleaning, we’ll tell you that before we start. We’d rather be straight with you than take your money on a roof that needs to come off anyway.
Ready to Get Rid of Those Black Streaks?
Vista Pro Exterior Cleaning has been soft washing roofs across Ozaukee, Milwaukee, Washington, and Sheboygan Counties for years. We follow ARMA recommended procedures, use professional grade solutions, and always run a test spot before treating the full roof.
Our roof cleaning service page has more detail on our process, or you can request a free quote instantly by clicking the button above. With nearly 400 five star reviews as of this writing, we’re the professional roof cleaning company you can count on here in the Greater Milwaukee and Sheboygan areas.
See Also:_____________________________________________________________________
Why Lichens Can Destroy Milwaukee Roofs
How Long Does The Roof Cleaning Process Take