Those dark streaks are **algae — specifically Gloeocapsa magma** — not dirt, mold, or wear. Houston’s humidity makes it nearly universal here.
What you’re seeing:
– **It feeds on limestone filler** in asphalt shingles, which is why it appears on shingles and not on metal.
– **It grows on north-facing slopes first,** where surfaces stay damp longest.
– **It streaks downward** because spores wash down with rain.
– **It’s usually cosmetic** in the near term, though heavy long-term growth retains moisture and can shorten shingle life.
How to deal with it:
– **Never pressure wash.** High pressure strips the granules that protect the shingle, taking years off the roof to fix a cosmetic problem.
– **Soft washing** with a manufacturer-approved cleaner is the correct method.
– **Zinc or copper strips** installed near the ridge release trace metal ions with each rain and suppress regrowth for years.
– **Algae-resistant shingles** have copper granules built in — worth specifying on your next roof in this climate.
If the streaking runs alongside curling edges or granule loss in the gutters, the algae isn’t your real problem. Worth an inspection.