I get some version of this call all the time: “My roof has these black stripes running down it. Do I need a new roof?” Usually the answer is no. What you’re looking at is almost always algae, and it can be cleaned off safely if it’s done the right way.
I’m Frank, and I own Professional Power Washing. We clean roofs across Anne Arundel, Howard and Baltimore County, and this is one of the questions I get asked the most. So here’s what those streaks are, why they show up on so many Maryland roofs, and how we clean them off without hurting your shingles.
What are the black streaks on my roof?
Those dark streaks are usually a type of algae called Gloeocapsa magma. The Asphalt Roofing Manufacturers Association (ARMA), the trade group for the companies that make shingles, calls it the most common of the algae that discolor roofs. It travels through the air, which is why you’ll often see a whole street where every roof has the same streaks.
ARMA says this algae is most widespread along the Eastern Seaboard in warm, humid conditions. That’s us. Between our humid summers and all the tree cover in places like Catonsville, Ellicott City and Severna Park, Maryland roofs give it plenty of shade and moisture to grow.
The streaks tend to run downhill because rain carries the algae down the slope, and they’re often worst on the north side of the house, which stays shaded and damp the longest.
Is roof algae damaging my shingles?
ARMA treats algae mainly as a discoloration problem. It’s ugly, and it makes a perfectly good roof look old and neglected, which matters a lot if you’re thinking about selling. But it’s not the same thing as a failing roof.
Moss is a different story. Moss is thicker, it holds water against the shingles, and it can lift them over time. If I see moss when I’m out at your house, I’ll point it out and tell you honestly whether I think it’s something a cleaning will handle or whether you should have a roofer look at it.
Can you power wash a roof?
Yes, a roof can be power washed, as long as it’s done at low pressure. Most people call any kind of exterior cleaning “power washing,” and that’s fine. What matters is how much pressure hits your shingles. A roof should be soft washed, which is the low-pressure kind of power washing. It should never be blasted with high pressure.
This is the part I really want homeowners to hear. Asphalt shingles are covered in small granules that protect them from the sun. Blast them with high pressure and those granules come right off. You can see them in the gutters afterward.
The shingle makers are clear about this. ARMA says high-pressure washing should not be used on asphalt roofing, and Owens Corning’s cleaning guidance says pressure washing and scrubbing will loosen and remove granules. Damaging the shingles that way can also put your manufacturer warranty at risk, so check your own warranty before anyone climbs up there with a pressure washer.
When someone offers to “power wash” a roof for cheap, that’s what they’re doing to it. The roof looks clean for a while and ages faster.
How we clean a roof instead
We soft wash roofs. That means low pressure, about what you’d get from a garden hose, and a cleaning solution that kills the algae instead of trying to blast it off. It’s the same approach the manufacturers describe: apply a bleach-based solution gently, let it sit long enough to work, then rinse gently.
The streaks often look noticeably better the same day, and they keep fading as the dead algae washes off in the next few rains. One customer told us her roof looked clean almost instantly. That’s not unusual, but on heavy growth I’d rather you expect a little fading time so you’re not surprised.
Before we start, I walk the property with you. I’ll look at your landscaping, any ponds or pools, where your downspouts drain and anything near the house you’re worried about, and I’ll explain what we’re going to do and why. When we’re done, we walk it again together so you can see the results with your own eyes.
Will the streaks come back?
Eventually, yes. I’m not going to tell you otherwise. ARMA describes cleaning as a temporary fix, because the algae spores are always in the air here. How fast it comes back depends on shade, how close your trees are, and which way your roof faces.
If you’re replacing your roof anyway, ask your roofer about algae-resistant shingles. If you’re keeping the roof you have, a soft wash every few years keeps it looking the way it should.
Can I clean my roof myself?
I’d honestly rather you didn’t. ARMA itself warns that a roof treated with a bleach solution gets slippery and dangerous. Add a ladder, a two-story house and a pressure washer, and that’s how people get hurt. If you’re set on doing it yourself, follow the manufacturer’s directions, stay off the roof, and never use high pressure.
Common questions about black streaks on roofs
Do black streaks mean I need a new roof?
Usually not. Algae streaks are a surface problem. If your shingles are curling, cracking or missing, that’s a roofing question. If they’re just streaked, a cleaning is usually what you need.
Is roof cleaning safe for my plants?
It can be, when it’s done carefully. I wrote a whole post on how we protect plants, pets and pools during a soft wash.
Can you clean my siding and gutters at the same time?
Yes, and most people do. Roof runoff ends up on your gutters and siding, so it makes sense to clean them together. We do house washing and gutter cleaning on the same visit all the time.
Want me to take a look?
If your roof has black streaks and you live in Anne Arundel, Howard or Baltimore County, give us a call at (410) 948-0228 or send us a message. I’ll tell you straight whether it needs cleaning, what it’ll take, and what kind of results to expect. We serve homeowners in towns like Owings Mills, Columbia and Pasadena.
Frank
Owner, Professional Power Washing
Sources: ARMA, Algae Discoloration of Roofs; Owens Corning, dark streak stain guidance.