What it is
Gloeocapsa magma is a single-celled cyanobacteria (sometimes called blue-green algae, though it's not technically algae). It's airborne, distributes regionally via wind, and colonizes any damp surface with calcium carbonate available — which is exactly what asphalt shingles are filled with. The dark pigmentation that makes the streaks visible is the bacteria's UV-protective sheath, similar to the way human skin produces melanin.
Why it damages roofs
- Eats the limestone filler. The calcium carbonate in shingles is the bacteria's food source. As it consumes the filler, the asphalt loses structural binding.
- Holds moisture against the shingle. The biofilm slows evaporation, keeping the shingle damp longer after every rain, which accelerates granule loss and freeze-thaw damage.
- Shortens shingle life by 5-7 years. Untreated colonization on a 30-year architectural shingle drops effective life to 22-25 years.
- Spreads. A streak on the north slope this year colonizes the entire north slope within 18-30 months.
ARMA-approved treatment
The Asphalt Roofing Manufacturers Association published guidance recommending a soft-wash chemical treatment using sodium hypochlorite. The standard protocol:
- Pre-wet plantings around the perimeter to dilute any over-spray.
- Apply a 50/50 mix from 12.5% pool-grade SH (working out to roughly 6% SH at the shingle), plus a surfactant for adherence.
- Dwell 15-20 minutes. Don't let the chemistry dry on the surface.
- Rinse with low-pressure water. No high-pressure ever — 60-200 PSI is the working range.
- Post-rinse plantings to neutralize any drift.
Visible improvement is gradual. The dead bacteria continues to wash off with subsequent rains for 2-4 weeks. Set this expectation in writing on the customer portal — it prevents callbacks asking why the roof "still looks streaked" the next day.
What not to do
- Pressure wash a shingle roof. Voids the manufacturer warranty. Strips granules. Cuts roof life by 5-10 years.
- Walk on a wet shingle roof. SH-wetted shingles are extremely slippery. Use a ladder + soft-wash extension, not foot traffic.
- Apply on a sunny 90°F day. Chemistry dries before it can dwell. Cloudy mornings or early evenings are ideal.
- Skip the surfactant. Without it, SH runs off the slope before it can kill the bacteria. No dwell = no cleaning.
Pricing reference
Standard residential roof soft wash runs about $0.45 per square foot of roof area, with a $450 minimum. A typical 2,000 sqft asphalt roof prices to $900. Higher pitches (8/12+), multi-story access, or heavy lichen presence add 15-30%. Roof wash is the highest-margin line in the cleaning bundle because the chemistry is cheap and the labor is one tech with one extension wand.
Mail roof-soft-wash postcards to streaked roofs in your area.
Free account, free rendering, $1 per mailed postcard. Render shows the same roof with the streaks gone.
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