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Those dark stains running down your shingles aren't dirt — they're a living algae called Gloeocapsa magma. Here's the correct, ARMA-approved method to safely remove them without damaging your roof.
If you've noticed dark gray or black streaks running down your roof — especially from the ridge toward the eaves — you're looking at Gloeocapsa magma, a type of cyanobacteria (commonly called algae) that thrives in warm, humid climates like South Florida.
Gloeocapsa magma travels through the air as spores and lands on rooftops. Once it settles, it feeds on the limestone filler used in asphalt shingles and moisture trapped under tile. The dark pigment you see is actually a UV-protective sheath the algae produces — its own sunscreen, essentially — which gives the streaks that distinctive dark black-brown color.
Florida's subtropical climate creates a perfect breeding ground for Gloeocapsa magma:
In northern states, homeowners might go 10–15 years before seeing algae growth. In Miami-Dade, Broward, and Palm Beach counties, most roofs show noticeable streaking within 3–5 years of installation — sometimes sooner on north-facing slopes that stay shaded.
The single biggest mistake homeowners make when dealing with roof algae is reaching for a pressure washer. It seems logical — blast the dirt off — but pressure washing a roof causes serious, expensive damage:
Soft washing uses a diluted sodium hypochlorite (bleach) solution applied at very low pressure to kill algae at the cellular level, then a gentle rinse to wash away the dead growth. This is the only method endorsed by ARMA for asphalt shingles and is safe for tile and metal roofs when applied correctly.
Before mixing any chemicals, walk around your home and assess the full extent of the staining. Identify whether you have algae (flat dark streaks), moss (greenish, raised), or lichen (crusty, gray-green patches). This guide focuses on algae. Note any damaged shingles, cracked tiles, or missing areas — do not walk on or treat a structurally compromised roof.
Also locate all downspouts and identify landscaping or plants within 10 feet of the house — these need to be pre-soaked and protected during treatment.
The ARMA-recommended treatment uses a sodium hypochlorite solution. For a standard residential roof job, mix the following in a 5-gallon pump sprayer or 12V soft wash tank:
This creates a roughly 5–6% active chlorine solution — strong enough to kill algae but safe for properly applied roof surfaces. Always wear chemical-resistant gloves, eye protection, and old clothing when mixing and applying.
Pre-wet all surrounding landscaping, shrubs, and grass with plain water from a garden hose. This dilutes any overspray and protects plant roots. Wet down the roof surface lightly as well if it is very hot (above 90°F) — this slows the solution from drying too quickly.
Apply the soft wash solution evenly across the stained areas using your sprayer set to its lowest pressure setting (under 100 PSI). Work from the ridge downward. Apply a thorough, generous coat — you want the solution visibly wet on the surface. Avoid spraying into open vents or directly at flashing seams.
Let the sodium hypochlorite solution sit on the roof surface for 15–20 minutes. This dwell time is critical — it allows the active chlorine to penetrate the algae's protective pigment sheath and kill the organism at the cellular level. Rushing this step results in superficial cleaning that won't prevent quick re-growth.
On hot days (common in Florida), monitor the solution — if it begins to dry or turn white, lightly mist the area with plain water to keep it wet and active. Do not let the solution dry completely before rinsing.
Using a standard garden hose with a spray nozzle on a gentle fan or shower setting, rinse the roof from the ridge downward. Work in sections, thoroughly flushing the solution and dead algae off the surface. Rinse gutters and downspouts as well, then give the surrounding landscaping a final thorough rinse with plain water.
Don't be alarmed if some dark staining remains immediately after rinsing — residual dead algae often lifts off gradually over the next 1–3 weeks with rainfall. The roof will continue brightening over time. A fully successful treatment may take 2–4 weeks to show its complete results.
The most common roof type in Florida. Follow the soft wash method exactly as described above. Be especially careful not to use any pressure — granule loss is irreversible. Most standard 3-tab and architectural shingles respond very well to a single soft wash treatment. Algae-resistant shingles (with copper granules) may be treated the same way but typically need cleaning less frequently.
Extremely common in South Florida, tile roofs can tolerate slightly more solution concentration but are even more pressure-sensitive than shingles. Never walk on or near a cracked tile. The soft wash solution is applied identically — the main difference is that tile roofs may also harbor moss in the grout lines, which requires a longer dwell time (25–30 minutes) or a second application. Do not use pressure to clean tile roofs.
Metal standing-seam and metal shingle roofs are increasingly popular in South Florida for hurricane resistance. Algae can grow on metal, particularly on coated steel or aluminum panels. Use a more diluted solution (25% sodium hypochlorite, 75% water) for metal roofs to avoid any risk of surface oxidation. Always rinse thoroughly. Metal roofs can generally tolerate a gentle rinse (up to 500 PSI, wide fan tip) but soft washing is still the safest approach.
DIY soft washing is feasible for single-story homes with accessible, low-slope roofs. However, there are clear situations where hiring a licensed professional is the safer and smarter choice:
We use ARMA-approved soft wash techniques with professional-grade 12V systems. Licensed, insured, and serving South Florida for 5+ years with a 4.9-star rating.
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