Those Black Streaks on Your Florida Roof Are Alive. Here's How to Kill Them.
Published April 1, 2026 · Krystal Klean Exterior
The dark streaks on Tampa Bay roofs aren't dirt — they're a living algae feeding on your shingles. Here's what it is and how to kill it without damage.
If your Tampa Bay roof has dark vertical streaks — almost black — running down from the ridge, that's not dirt. It's a living algae called Gloeocapsa magma, and it's eating your roof.
What is Gloeocapsa magma?
A cyanobacteria (blue-green algae) that feeds on the limestone filler in asphalt shingles. It thrives in Florida's heat and humidity. It spreads via airborne spores — which is why you'll see it on one house, then a few years later on every roof on the street.
Why it matters
- It shortens roof life. Algae eating limestone filler means shingles lose granules faster — up to 10 years off your roof's usable life in bad cases.
- It reduces energy efficiency. Dark streaks absorb heat. Attics run hotter. AC bills go up.
- It signals problems to buyers. A streaked roof tells a homebuyer "this home has been neglected" even if the rest of the house is pristine.
- It spreads. Left alone, it gets worse every year.
What NOT to do
- Don't pressure wash your roof. High pressure on shingles blows off the granules that protect the asphalt. It voids manufacturer warranties. It makes the problem worse long-term.
- Don't use bleach alone. Undiluted chlorine damages landscaping and drips stain siding.
- Don't ignore it. It spreads. The longer you wait, the more it costs.
What works: soft washing with algaecide
A professional soft wash uses low pressure (garden-hose level) plus a carefully mixed algaecide solution that kills Gloeocapsa magma at the root. It meets the ARMA (Asphalt Roofing Manufacturers Association) standard. Every major shingle brand (GAF, Owens Corning, CertainTeed) explicitly endorses this approach.
What to expect during a soft wash:
- Landscaping pre-soaked with water, often tarped
- Algaecide solution applied to the roof
- Short dwell time while solution kills the algae
- Gentle rinse with low pressure
- Landscaping rinsed and double-checked
- Results often get better over the next 1–2 weeks as dead algae fully releases
How long results last
3 to 5 years typically. We offer a 2-year spot-clean warranty on shingle roofs.
Cost
A typical Tampa Bay shingle roof runs $399 to $1,495 depending on size, pitch and access. We price free.
→ See our full roof cleaning process or call 727-579-7825.