Why Central Texas roofs get black streaks (and how to remove them)
What those black streaks on your shingles actually are, why they spread, and the right way to clean them without damaging the roof.
3 min read
It is not dirt
The dark streaks running down many asphalt shingle roofs are a type of cyanobacteria, commonly identified as Gloeocapsa magma. It feeds on the limestone filler used in many shingles and builds a dark protective coating that shows up as streaks.
Why it spreads
Spores travel on the wind and with animals, so once one roof in a neighborhood has it, nearby roofs often follow. North-facing slopes and roofs shaded by trees stay damp longer and tend to show it first.
Does it hurt the roof?
The streaks are mostly a cosmetic problem, but heavier growth like lichen and moss holds moisture against the shingles. Dark staining can also make a roof absorb more heat.
How to remove it safely
Do not pressure wash shingles. The Asphalt Roofing Manufacturers Association warns that high pressure can remove the granules that protect shingles from sunlight. A low-pressure soft wash solution kills the growth, and remaining staining continues to fade over the next several rains.
Roof work also involves fall risk. Hiring an insured company that does this regularly is safer than doing it from a ladder.