Direct Answer
Those black streaks are algae — specifically Gloeocapsa magma — feeding on the limestone filler in modern asphalt shingles. It's common on humid, shaded roofs in the Carolinas. It's cosmetic at first, but it holds moisture against the shingles and can shorten roof life. It's removed with a low-pressure chemical treatment, not pressure washing.
The algae spread down the roof slope over time, fed by the limestone in the shingles and the humid Carolina climate. It usually starts on north-facing or shaded slopes that stay damp longer.
What to Do About It
- Don't pressure wash — it strips granules and ruins shingles
- Use a low-pressure chemical treatment (soft wash) that kills the algae
- Trim branches to reduce shade and moisture
- Consider algae-resistant shingles when replacing
Signs & Symptoms
- Dark streaks running down the roof from the peak
- Black or dark-green patches on shaded slopes
- Streaks worsening over months
Repair Options
- Soft-wash chemical treatment to kill the algae
- Trim branches to reduce shade
- Algae-resistant shingles on replacement
When to Call a Roofer
If the streaks bother you or are spreading, a professional soft-wash treatment restores the roof safely. Don't pressure wash.
Local Considerations — Upstate SC & Western NC
Upstate SC and Western NC's humidity makes algae growth very common. North-facing and shaded slopes are most affected. Periodic treatment keeps it in check.
How Beyond the Ladder Handles It
We treat roofs with safe, low-pressure methods that kill algae without damaging shingles, and recommend prevention for your roof's exposure.
Related Questions
Are the black streaks damaging my roof?
Initially cosmetic, but algae holds moisture against the shingles, which can shorten their life. Removing it protects both appearance and longevity.
Can I pressure wash the streaks off?
No. Pressure washing strips the protective granules off asphalt shingles and shortens roof life. Use a low-pressure chemical treatment instead.