The black streaks running down your roof are a living organism: Gloeocapsa magma, an airborne algae that lands on roofs and feeds on the crushed limestone filler inside asphalt shingles. It isn't dirt, it doesn't wash off in the rain, and it doesn't stop on its own — the streaks are the algae colony spreading downhill from wherever the spores first took hold.
It shows up as dark vertical streaks, usually worst on north-facing slopes and shaded sections where the roof stays damp longest. If your neighbors have it too, that's no coincidence — the spores travel on the wind from roof to roof.
Is it actually hurting the roof?
Yes, slowly. The algae feeds on the limestone in the shingles, and the dark colonies hold moisture against the roof and absorb heat — meaning hotter shingles through an Austin summer and faster granule loss. Left alone for years, it shortens shingle life. It also flattens your home's curb appeal and is one of the first things a buyer's inspector will flag.
Why Austin roofs get it so badly
Gloeocapsa magma thrives on humidity and warmth, and Central Texas supplies both. Our long, humid springs give colonies a head start, and asphalt shingles here use plenty of the limestone filler the algae eats. Once one roof on the street has visible streaks, spores spread through the neighborhood every windy day.
How to remove it — and how not to
The safe removal method is soft washing: a low-pressure application of cleaning solutions that kills the algae completely, followed by a gentle rinse. Streaks fade immediately and keep fading over the following weeks as the dead growth releases and washes away with normal weather.
What you should never do is pressure wash a shingle roof. High pressure strips the protective granules that give shingles their weather resistance and fire rating, and it voids most manufacturer warranties. If a contractor proposes pressure washing your roof, get a different contractor.
Keeping it from coming back
A proper soft wash kills the colony rather than trimming it, so most Austin roofs stay streak-free for several years. Trimming back branches that shade the roof helps it dry faster and slows regrowth. When it eventually returns — airborne spores guarantee it someday will — a maintenance wash is far quicker than the first restoration.