Those streaks are usually algae, not dirt or soot. It grows on the shingles themselves, mostly on the slopes that stay damp the longest.
From the driveway it’s easy to take for grime or tree droppings. It isn’t, and that changes how it should come off.
Why it grows here, and why it spreads
Evansville gives algae plenty of damp to work with. Our mean relative humidity runs about 70.2% a year, peaking in December and September (NOAA 1961-1990 data).
Once streaks start, they keep growing wherever the shingles stay wet. And neighbors share the same trees, shade and weather. If the house next door has streaks, the same conditions are at work on your roof too.
Why shady slopes go first
Algae needs moisture to grow. A slope that gets less sun, often the north side, stays damp longer after rain and morning dew. Trees hanging over the roof add shade and drop leaves that hold water. Those are the spots where streaks tend to start.
How it comes off without pressure
Not with a pressure washer or a stiff brush. Either one can strip the granules that shield the asphalt from UV (InterNACHI). Our roof cleaning is a low-pressure soft wash, never a pressure wash. A cleaning solution does the work, and a gentle rinse carries it off.
One catch. Algae can grow back over time, especially on the slopes that stay damp. That’s why some owners add roof protection after a cleaning, to help the roof stay clean longer.
Want us to look at your streaks up close? Call or text (812) 664-6222 to book a free exterior evaluation.