You noticed them the first time you pulled into the driveway last fall. Dark vertical stripes running down the north side of your roof, concentrated under overhangs, spreading a little further every year. You probably figured it was dirt, or maybe soot from the chimney. It isn't.

Those streaks are a living organism called Gloeocapsa magma — a cyanobacteria that feeds on the limestone filler in asphalt shingles. It's been on roofs for centuries, but it's gotten worse over the last 20 years because manufacturers started adding more crushed limestone to shingles as a cost-saving measure.

Why you can't pressure-wash it off

Instinct says: high pressure will blast it. And it will — along with the granule layer that makes your shingles waterproof and UV-resistant. We see the damage a few times a year from homeowners (or, sadly, other contractors) who took a wand to a roof. It looks clean for a month. Then the bare spots start flaking off.

"A pressure washer on a roof is like a belt sander on a fine veneer. Technically it works. Once."

The correct method is a soft wash: a low-pressure application of a sodium hypochlorite and surfactant mix that kills the organism at the root. No scrubbing, no granule loss. Rain does the rest over the following weeks.

What to do about it

If the streaks are minor and you're handy, you can buy a pump sprayer and a 50/50 bleach-and-water mix at any hardware store. If you do it yourself, remember:

  • Wet your landscaping before you start. Chlorine kills plants.
  • Never apply on a sunny afternoon — the mix flash-evaporates.
  • Rinse the gutters thoroughly when you're done.
  • If you see streaks returning within 18 months, the application wasn't strong enough.

If your roof is steep, tall, or the streaks are heavy, call someone. This isn't one to learn on. We do roofs from the ground — nobody walks the shingles.

When to call a pro

Four conditions where DIY isn't worth the risk:

  1. Two or more stories. The fall risk alone isn't worth a $400 job.
  2. Heavy colonization. If the streaks are more black than gray, the organism has rooted into the granule layer and needs a stronger mix than off-the-shelf.
  3. Around solar panels, skylights, or HVAC. Mis-application can void warranties and stain surrounding siding.
  4. HOA requirements. Some HOAs require licensed contractors and a Certificate of Insurance on file.

A ground-based soft wash from a reputable crew runs $400–$699 for a single-family roof in North Georgia, takes two to four hours, and keeps streaks away for three to five years. Get a quote or text us a photo at (404) 668-5522.