Drive through any Lincoln neighborhood that's fifteen or twenty years old and you'll see them: long, dark streaks running down shingle roofs, usually worst on the north side, like someone dragged a dirty mop from ridge to gutter. Most homeowners assume it's soot, dirt, or just an aging roof. It isn't. Your roof has a biology problem, and the fix is gentler than you'd think.
The streaks are alive — meet Gloeocapsa magma.
Gloeocapsa magma is an airborne cyanobacteria — a blue-green algae — and it lands on roofs the same way pollen lands on your car. Modern asphalt shingles give it a buffet: manufacturers use crushed limestone as a filler in the shingle mat, and limestone happens to be exactly what this organism eats. Once it settles in, it grows a dark, UV-blocking outer sheath to protect itself from the sun. That dark sheath is the black streak. The stains run vertically because every rain carries the colony a little further down the slope, seeding new growth as it goes.
So the streaks aren't stains sitting on your roof — they're a living colony feeding on it.
A quick way to confirm you're looking at algae rather than ordinary grime: the streaks start below the ridge, widen as they run toward the gutter, and skip the metal. You'll often notice a clean, streak-free stripe directly below flashing, vents, or a chimney cap — trace metal washing down the slope keeps the organism from taking hold there. Hold that thought, because that clean stripe is also the secret behind the best prevention trick on the list below.
Why Nebraska roofs feed it so well.
This algae used to be mostly a Gulf Coast problem. It has spent decades moving north, and eastern Nebraska suits it fine: muggy Julys, heavy morning dew, and long stretches where the shingles never fully dry out. Shade makes it worse — that's why the streaks show up first on north-facing slopes and under overhanging trees, where moisture lingers longest. And because the spores travel on the wind, one streaked roof tends to seed the neighbors. If two houses on your street have it, yours is on the list.
It's not just cosmetic, either. The colony holds moisture against the shingle, eats the limestone filler out of the mat, retains heat, and lays out a welcome mat for moss and lichen — which physically lift shingles as they grow. Left alone for years, it shortens the working life of the roof.
Why you should never pressure wash a shingle roof.
Here's the part that saves people real money. The gravelly granules on top of each shingle are its armor — they're what stands between the asphalt underneath and the sun. A pressure washer strips those granules off by the pound. A few passes with high pressure can take years of natural wear off a roof in one afternoon, and it drives water up under the shingle laps while it's at it.
Shingle manufacturers are explicit about this: high-pressure washing damages the roof surface and can void your shingle warranty. Industry guidance from asphalt roofing manufacturers calls for low-pressure chemical cleaning only — never a pressure washer.
So if a company shows up to quote your streaky roof and unloads a surface cleaner and a 4,000 PSI machine, close the door. That's not cleaning, that's sanding.
What actually works: soft washing.
Roof soft washing flips the equation — chemistry does the work instead of pressure. We apply a detergent blend, built around sodium hypochlorite and a surfactant, at little more than garden-hose pressure. It sits on the roof, soaks into the colony, and kills the organism at the root — not just the visible sheath. Then a gentle rinse carries it away. Granules stay put, laps stay sealed, warranty stays intact.
Heavier streaking fades immediately and keeps improving over the following weeks as rain rinses the dead material through. Because the treatment kills the algae rather than shaving the top off it, the roof stays clean far longer than anything pressure could accomplish. Our roof soft wash starts at $399 — against the cost of replacing a roof early, it's one of the cheapest pieces of maintenance a house can buy.
Keeping the streaks from coming back.
- Zinc or copper strips at the ridge — every rain washes trace metal ions down the slope, and the algae hates them
- Trim overhanging branches — more sun, faster drying, less algae
- Keep gutters flowing — standing debris holds the moisture the colony needs (our gutter brightening pairs well here)
- Re-treat every few years — a periodic soft wash keeps a new colony from ever establishing
We soft wash roofs across Lincoln and the surrounding towns — Waverly, Hickman, Eagle, Crete and everywhere within about 30 miles. Before-and-after photos on every job, and you approve the results before we leave.