How to Remove Algae and Black Streaks from Your Roof

The dark streaks on asphalt shingle roofs are almost universally caused by Gloeocapsa Magma — an airborne cyanobacteria (a type of algae) that feeds on the limestone granule filler in asphalt shingles. It starts as small spots and spreads as streaks that run down the roof slope, following water flow.

Green patches are moss or lichen — different organisms, different treatment approach, and a more urgent problem. Moss root systems physically lift and separate shingles. Lichen bonds to the shingle surface with a chemical bond that makes it harder to remove and leaves surface pitting if left for years.

Understanding which you have matters because the treatment and urgency differ. Streaks alone (algae) are primarily cosmetic in the early stages. Visible moss or lichen growth needs prompt attention.

Algae is cosmetic initially — but long-term, it retains moisture against shingles and accelerates granule loss. Many shingle manufacturers explicitly note algae as a contributing factor in premature shingle failure, and some warranties require treatment when algae coverage reaches certain thresholds.

Moss and lichen are structural concerns. Heavy moss coverage causes physical lifting of shingles and dramatically increases the amount of water trapped against the roof deck. Lichen literally etches into the shingle surface over time. If you see significant green growth, schedule cleaning within the current season — not next year.

The Asphalt Roofing Manufacturers Association (ARMA) recommends a diluted sodium hypochlorite (bleach) solution applied at low pressure as the industry-standard removal method. This kills algae and moss at the root and washes clear in the next rain.

What not to do: pressure washing shingles at high PSI blasts granules off the shingle surface — those granules are the primary UV protection layer and cannot be replaced. A single high-pressure wash can strip years of life from a roof. Any contractor who proposes pressure washing your asphalt shingles should be declined.

For stubborn lichen, professional cleaners use higher-concentration bleach solutions with longer dwell times. Some lichen takes 1–2 rainfalls after treatment to fully release and wash away — this is normal.

Zinc and copper ridge strips are the most effective long-term prevention. As rainwater runs over the metal, it picks up metal ions that inhibit algae and moss growth over the entire roof below the strip. Each set typically provides 3–5 years of effective protection.