Roof Algae (Black Streaks)

The dark brown or black streaks that run down many light-coloured roofs are usually a growth of cyanobacteria (blue-green algae), most often identified as Gloeocapsa magma. The streaks are common on asphalt shingles in warm, humid regions, but they appear in many climates and also on tile and other surfaces.

How it grows

Spores are carried by wind and by birds and small animals from roof to roof, which is why streaking often spreads through a neighbourhood. The organism thrives on surfaces that are warmed by sun and dampened by rain or dew, and it is often reported to feed on the limestone filler used in many asphalt shingles. As it grows it produces a dark protective pigment against ultraviolet light; the visible streaks are largely accumulated layers of this dark material and dead cells. Streaks usually start on the slopes that stay damp longest and follow the path of runoff.

Effects

The main effect is appearance. Some sources note that the dark layer absorbs more heat, and that long-term growth may contribute to surface wear. Algae is different from moss and lichen, which are thicker growths that hold more moisture and can lift roof units.

Cleaning

The usual recommendation from shingle manufacturers' associations is a low-pressure application of a diluted sodium hypochlorite (bleach) solution, followed by rinsing, rather than pressure washing. Plants below should be protected and the roof surface kept safe to walk on. Commercial cleaners and professional soft washing are alternatives (see roof cleaning).

Prevention

• Algae-resistant shingles, sold in many markets, contain a proportion of granules treated with copper compounds that slowly release ions which inhibit growth. The copper industry reports protection periods of up to about 30 years depending on the product.
• Copper or zinc strips near the ridge release metal ions with rain.
• Reducing shade and moisture by trimming trees helps the roof dry faster.

Algae streaks are generally not covered as damage by insurance, and algae-resistance warranties vary by manufacturer and region.