Why does my roof have black streaks? (Atlanta edition)
Short version: it's algae. Specifically, gloeocapsa magma. Long version: it's eating your shingles, and you probably shouldn't pressure-wash it.
If you've driven through any metro Atlanta neighborhood in the last five years, you've seen them — long black streaks running down asphalt-shingle roofs, almost always heaviest on the north-facing side. Most homeowners blame "dirt" or "weathering." It's neither.
It's a living organism. And it's worse here than almost anywhere else in the country.
What you're actually looking at
The black streaks are gloeocapsa magma, a hardy blue-green algae. It lives on top of asphalt shingles and quietly feeds on the limestone filler that's used to weight and texture the shingle granules. To survive the brutal Atlanta sun, it secretes a dark protective sheath — that's where the black color comes from.
The spores travel through the air. One streaked roof in a neighborhood becomes ten streaked roofs over the next few years. Birds carry it. Wind carries it. Once it lands and finds a damp, shaded surface, it sets up shop.
Why it's worse in metro Atlanta
Three reasons we get hit harder than most cities:
- Humidity. Atlanta summers are 70–90% humidity for months. Algae loves moisture.
- Tree canopy. Beautiful old neighborhoods like Buckhead, Sandy Springs, and Marietta have thick canopy. Shade keeps roofs damp longer, which is exactly what gloeocapsa needs.
- Long warm season. March through November is growing weather. Northern cities get a hard freeze that knocks the algae back. We don't.
Is it just cosmetic?
No. Three things are happening under the streaks:
- The algae is literally eating the limestone inside your shingle granules. Granules protect the asphalt underneath from UV. Less granule = faster shingle failure.
- The black sheath absorbs heat instead of reflecting it. That raises attic temperatures, which raises cooling bills.
- Manufacturers like GAF, CertainTeed, and Owens Corning estimate algae shortens shingle life by 30 to 50 percent if left untreated.
A new roof in Atlanta costs $15,000 to $35,000 depending on size and material. A roof soft wash costs $700 to $1,000 and adds years of life. The math isn't close.
The biggest mistake homeowners make
Never let anyone pressure-wash an asphalt shingle roof. Direct high-pressure water on shingles strips the protective granules off, voids most manufacturer warranties (read your warranty — most explicitly prohibit pressure washing), forces water under the shingles, and accelerates the damage you were trying to fix.
If a roof-cleaning company quotes you a "roof power wash" or "pressure wash for shingles" — they're either uninformed or hoping you are. Walk away.
The right way: ARMA-approved soft wash
The Asphalt Roofing Manufacturers Association (ARMA) — the industry body for shingle manufacturers — publishes a specific cleaning standard. It's a low-pressure (under 100 PSI, often closer to garden-hose pressure), chemistry-first method using a sodium hypochlorite solution that kills the algae at the cellular level rather than blasting it off the surface.
The solution is applied, allowed to dwell, then rinsed gently. The algae dies, the streaks vanish, and the shingles aren't disturbed. We've been doing it the ARMA way at Sunshine since 2004 — every roof, every time.
How long does it last?
Properly executed, a roof soft wash kills the algae at the root. Visible re-growth on a typical north-facing Atlanta roof takes three to five years. Sunshine includes a 3-year warranty against visible algae re-growth on every roof soft wash we perform — if streaks show up inside three years, we come back and retreat at no charge.
What it costs in metro Atlanta
Real numbers from our jobs (range, not flat):
- Single-story home, moderate algae: $500 – $750
- Two-story home, moderate algae: $700 – $1,000
- Large home or heavy algae infestation: $1,000 – $1,500
- Add gutter cleaning while we're up there: +$150 – $300 (bundled — saves a separate trip)
Most customers in Buckhead, Alpharetta, Sandy Springs, and Milton land in the $700–$1,000 range.
What to do next
If you're looking at black streaks right now, three options:
- Ignore it. You'll trade a $1,000 soft wash today for a $20,000 roof replacement in 8 years. Not the move.
- Hire the cheapest pressure-washing guy. Voids your warranty, shortens your roof life, and the streaks will be back within a year. Worse than ignoring it.
- Get an ARMA-approved soft wash from someone who's been doing it locally for 20+ years. Streaks gone, shingles untouched, 3-year warranty. That's what we do.
Want a free quote? Call Mark at (678) 458-6305 or use the instant quote form. We service all 22 metro Atlanta cities — Buckhead, Alpharetta, Sandy Springs, Milton, Johns Creek, Marietta, Roswell, and beyond.