Ever looked up at your roof and noticed dark, ugly streaks running down the shingles—like your house just had a rough day in a rainstorm it couldn’t shake?
You’re not alone.
We get a lot of calls at BC Exteriors from homeowners across Michigan asking what those black streaks are, if they’re dangerous, and—most importantly—how to get rid of them.
Here’s the truth: they’re not just dirt.
What Are the Streaks?
Those black streaks you see are usually caused by a type of airborne algae called Gloeocapsa Magma.
Yep—algae. Not mold, not moss, and not just dirt from rainwater runoff.
This stuff thrives in humid environments (hello, Michigan summers) and loves to feed on the limestone filler found in many asphalt shingles. Once it takes hold, it can spread quickly—especially on the north side of your roof where there’s more shade and moisture.
Is It Damaging My Roof?
In short: it can be.
While algae alone isn’t the fastest roof destroyer, it’s not something to ignore. Over time, those streaks:
- Trap moisture, which can shorten your roof’s lifespan
- Make shingles less reflective, increasing heat buildup and cooling costs
- Lower your curb appeal (especially important if you’re thinking about selling)
- Lead to the growth of other, more damaging organisms like lichen or moss
If left untreated, that pretty roof of yours can go from “slightly streaky” to “full-on biohazard zone.”
Can I Just Clean It Off?
Yes—but carefully.
Power washing your roof might seem like a good idea, but it can actually do more harm than good. It’s easy to blast off granules, damage shingles, or void your manufacturer’s warranty. The better route is using a gentle, low-pressure soft wash system and an algae-killing solution made specifically for roofing materials.
At BC Exteriors, we always recommend a professional inspection before taking action. In some cases, a cleaning is all you need. In others, those streaks are a sign of deeper issues—like granule loss, poor ventilation, or moisture damage underneath.
How Do I Prevent It?
- Trim back overhanging trees to reduce shade and moisture retention
- Keep gutters clear to promote proper drainage
- Get a regular roof inspection (We do those, by the way )
Let’s Keep Your Roof Looking—and Working—Its Best
Whether your roof is five years old or 25, those streaks are a signal worth paying attention to. And as your local, veteran-owned roofing experts, we’re here to help you make sense of what’s up there—so you can make smart decisions down here.
Schedule a roof inspection with us by filling out the form on our homepage or calling 248-688-0211. We’ll let you know exactly what you’re dealing with and what your best options are—no pressure, no guesswork.
Because when it comes to your home, knowledge is protection.
And we’ve got you covered—literally.