The black streaks running down asphalt shingle roofs across northern Ohio are almost always Gloeocapsa magma — an airborne algae that feeds on the limestone filler inside the shingle. It's not just cosmetic; it shortens roof life and raises attic temperatures.
What causes the black streaks
Asphalt shingles aren't pure asphalt. Modern shingles contain a limestone filler that gives them body and flexibility. That filler is also food for Gloeocapsa magma, a type of blue-green algae that travels through the air and lands on roofs.
Once it lands, it spreads. The algae feeds on the limestone, and as it grows it produces a dark pigment that protects it from UV — that's the black you see. The streaks run down the roof because rain carries the algae and its pigment down the slope, creating the signature drip pattern.
It's not dirt, and it's not mold. It's a living organism growing on the shingle.
Why northern Ohio roofs are vulnerable
Gloeocapsa thrives in humid, shaded conditions. Northern Ohio gets plenty of both. Lake-effect moisture, mature tree cover over many Lorain County neighborhoods, and long stretches of overcast weather give the algae what it needs.
North-facing roof slopes are usually hit first because they dry slowest. Roofs under heavy tree cover follow. Once the algae takes hold on one roof, spores spread to nearby homes, which is why whole streets often show the same streaks.
Why you should never pressure wash a roof
A pressure washer will remove the algae. It will also remove the granules that protect the shingle from UV and weather. Asphalt shingles are not designed to take direct pressure, and once granules are gone they don't come back. The roof ages faster after a pressure wash than before it.
Pressure washing can also force water up under the shingle tabs, which is the opposite of what a roof is supposed to do. The short-term cosmetic gain isn't worth the long-term damage.
How soft washing removes algae safely
Soft washing is the right method for roofs. It uses a low-pressure application of a cleaning solution that kills the algae at the surface, followed by a gentle rinse. The solution does the work — not pressure.
The process:
- Protect landscaping and catch runoff at the downspouts.
- Apply an algae-targeted cleaning solution at low pressure.
- Let it dwell long enough to kill the algae and loosen the pigment.
- Rinse with low-pressure water.
- Flush downspouts and landscaping areas.
The streaks lighten as the algae dies, and the roof continues to shed the remaining pigment over the next several rains. It's a slower visual result than blasting, but it doesn't shorten the roof's life.
If you're dealing with this, our soft washing in North Ridgeville, Ohio page covers how we approach a roof visit, and our roof soft washing page explains the method in more detail.
Keeping algae from coming back
Algae will eventually return — it's airborne — but you can slow it down. Trim back branches that shade the roof, keep gutters clear so water doesn't sit, and consider a zinc or copper strip near the ridge. As rain washes over the strip, trace amounts of metal run down the roof and inhibit algae growth.
A clean roof also stays cooler. Dark algae growth absorbs heat, which raises attic temperatures and cooling costs. Removing it is one of the few exterior projects that genuinely helps the home beneath it.