What exactly is Gloeocapsa magma?
Gloeocapsa magma is a species of cyanobacteria, a photosynthetic bacteria often called blue-green algae. It travels through the air as spores, lands on asphalt shingle roofs, and feeds on the crushed limestone filler inside the shingles. As colonies mature, they develop a dark protective outer sheath, which is what creates the black streaks visible from the ground.
Although commonly called "roof algae," it is technically a bacteria capable of photosynthesis. The distinction matters less for homeowners than the behavior: it is a living, spreading organism, not a stain.
This is not a new phenomenon, either. Algal discoloration of roofs across the United States and Canada was documented in the scientific literature as far back as Brook's 1968 report in the Journal of Phycology, decades before black streaks became the widespread suburban complaint they are today.
Why does it turn black?
Young Gloeocapsa magma colonies are not visible from the ground. As a colony grows and becomes exposed to ultraviolet light, it produces a dark pigmented sheath that acts like a sunscreen, protecting the cells underneath from UV radiation. That pigment is what you see as black streaks. By the time streaks are visible, the colony has typically been established for two or more years.
How does it spread from roof to roof?
The spores are airborne. Wind carries them from established colonies to nearby roofs, which is why streaking often moves through a neighborhood over several years. Once a spore lands on a shingle, it needs three things to establish a colony: moisture, warmth, and food. Humid climates like the Southeast provide the first two, and the limestone filler in modern asphalt shingles provides the third.
This is also why the problem compounds: the more streaked roofs in a neighborhood, the more spores are in the local air supply, and the faster clean roofs become colonized. You can read more about the conditions that accelerate growth in our guide to the variables that determine how fast roof algae spreads.
Why does it start on north-facing slopes?
North-facing roof slopes receive the least direct sunlight, so they stay damp longer after rain and morning dew. That extended moisture window gives spores the best chance to attach and multiply. In most cases, black streaks appear on the north slope first, then spread to shaded east and west slopes, and finally to the sunniest south-facing sections.
Why do modern shingles feed it?
Decades ago, asphalt shingles were made with more asphalt and felt. Modern architectural shingles use crushed limestone as a filler to add weight and fire resistance. Limestone is calcium carbonate, and Gloeocapsa magma consumes it as a nutrient source. In effect, the organism is slowly eating the shingle itself, which is why heavy, long-standing colonization contributes to granule loss and premature shingle aging.
It Is Not Just Cosmetic
What actually kills it?
A solution of sodium hypochlorite and water, applied at low pressure and allowed to dwell on the surface, kills Gloeocapsa magma down to the root of the colony. This is the basis of the ARMA-approved soft wash method. Pressure washing, by contrast, blasts the visible pigment off the surface without killing the organism, while stripping away protective granules and voiding manufacturer warranties.
Prevention options like algae-resistant shingles and zinc or copper strips use copper ions to slow recolonization, though neither eliminates the organism entirely.