Guide · Arlington Heights, IL
Black streaks on your roof: what they are and how to get rid of them
Why those dark stripes appear, whether they damage your shingles, and how soft washing removes them.
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Short answer
The black streaks on most asphalt shingle roofs are a type of algae, usually Gloeocapsa magma. It isn't dirt, soot or mold from inside the house. It spreads from roof to roof on wind and rain, grows best on shaded, damp slopes, and can be removed safely with a low-pressure soft wash.
What causes black streaks on a roof
The algae behind roof streaks is naturally blue-green. The dark color comes from a protective coating it builds up against the sun, which is why the streaks look black from the street.
It grows where three things come together:
- Moisture. Roofs that stay damp after rain or morning dew.
- Shade. North-facing slopes and roofs under mature trees dry out slowest.
- Something to feed on. Many shingles contain limestone filler, and the algae feeds on it.
The streaks follow the path water takes down the roof, which is why they form long vertical stripes. And because the spores travel on the wind, once one roof on a street has it, the neighbors' roofs often follow.
Is it damaging your roof?
The streaks start as a cosmetic problem, but they don't stay that way:
- Dark streaks absorb more heat, which can add stress to the shingles over time.
- Algae holds moisture against the roof surface.
- Where algae grows, moss and lichen often follow, and those can lift shingle edges and grip the surface hard.
Streaks can also make a roof look older than it is, which matters if you're thinking about selling.
Why pressure washing makes it worse
It's tempting to blast the streaks off, but pressure washing a roof causes more problems than it solves. It strips the granules that protect shingles from the sun, can force water under the shingles, and leaves the root of the algae in place, so the streaks come back. Shingle manufacturers advise against it for exactly these reasons. Soft wash vs pressure wash.
How soft washing removes roof streaks
Soft washing treats the algae instead of trying to force it off:
- Plants and surfaces below the roofline are wetted down to protect them.
- A cleaning solution is applied to the roof at low pressure.
- The solution kills the algae, along with any moss and lichen.
- The roof and everything below it are rinsed gently.
Most of the streaking disappears on the day. Any remaining shadow usually fades over the following rains as the dead growth washes away.
How to slow it from coming back
- Trim overhanging branches so the roof gets more sun and dries faster.
- Keep gutters flowing so water doesn't sit along the roof edge.
- Consider zinc or copper strips near the ridge. Rain carries trace metal down the roof and helps discourage regrowth.
- Choose algae-resistant shingles when it's time to replace the roof.
FAQ
Is the black stuff on my roof mold?
Usually not. On asphalt shingle roofs it's almost always algae.
Will bleach from the hardware store fix it?
Mixing and applying it safely on a roof, while protecting plants and yourself, is harder than it sounds. A professional soft wash is safer for you and the roof.
How long until the streaks come back?
It depends on shade and tree cover. After a soft wash kills the algae, most roofs stay clean for years, and sunny roofs stay clean longest.
Get the streaks off your roof
Our professional roof cleaning removes black streaks, moss and lichen at low pressure, anywhere in Arlington Heights and nearby towns. Call any time, day or night.
Cleaner homes, brighter neighbors
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