The vertical black streaks you see on Houston roofs have a name: Gloeocapsa magma. It's a cyanobacteria, and it's been eating the limestone filler in your shingles.
Why it shows up here
Houston's humidity plus tree cover plus asphalt shingles is almost a perfect environment for it. North-facing slopes show it first because they stay damp longest after rain.
Why you can't pressure-wash it off
The streaks are on the surface, but the organism is rooted into the granules. Pressure-washing strips those granules — which is what protects your shingles from UV. You trade a cosmetic problem for a structural one.
What actually works
Low-pressure soft-wash with sodium hypochlorite at the right dilution. It's what the Asphalt Roofing Manufacturers Association endorses, it kills the organism at the root, and it keeps your warranty intact. Done properly, you get 2–4 years before visible regrowth in Houston.
