What Is Gloeocapsa Magma?

No. Black roof streaks are almost universally caused by Gloeocapsa magma cyanobacterium, not mold. While roof mold can exist in sheltered areas with organic debris, the characteristic vertical dark streaks visible from the street are caused by this specific photosynthetic organism.

On Vancouver Island, a new colony can establish and become visibly streaky within 1–2 wet seasons (approximately 18 months). Once established, the organism spreads to neighbouring roofs via wind and birds within 2–3 additional seasons.

Yes, but more slowly. Metal roofs (particularly standing seam) have less surface texture for the organism to anchor to, and metal ions (zinc, galvanised steel) provide some natural inhibition. However, paint-coated metal panels in shaded areas can develop algae streaking within 3–5 years.

Cedar shake roofs develop algae streaking but Gloeocapsa magma is less dominant on wood surfaces — moss and lichen are more prevalent concerns. Cedar's natural oils provide some resistance, but treated or weathered cedar develops algae streaks, particularly in shaded north-facing valleys.

No direct human health risk. The organism is not pathogenic and does not produce toxins harmful to humans under normal roof exposure conditions. The risk is entirely structural — to the roofing materials themselves and, by extension, the building envelope below.