Black streaks running down roof slopes are one of the most common exterior issues in Greater Victoria. They are not dirt, soot, or water staining. They are caused by blue-green algae (Gloeocapsa magma) — a microorganism that feeds on the limestone filler in asphalt shingles.
Gloeocapsa magma is an airborne cyanobacterium. Spores land on the roof and feed on calcium carbonate — the limestone filler used in modern asphalt shingles. The bacteria spread as rain washes spores down the slope, creating the dark streaks.
The streaks are most prominent on north-facing slopes and in shaded areas where the roof stays damp longer. Victoria's coastal climate — frequent rain, morning dew, and high humidity — creates ideal conditions for this algae to thrive.
Because the algae is embedded in the shingle surface and actively feeding on the material, water alone will not remove it. Pressure washing would blast the algae — and the shingle granules — off the roof.
Soft wash treatment kills the algae at the source. The streaks lighten and release through subsequent rain exposure. No scrubbing. No high pressure. No shingle damage.
TREAT IT. DON'T BLAST IT.
A controlled treatment solution is applied at low pressure, targeting the algae. The organism dies, the discolouration fades, and natural weathering clears the surface over the following weeks.
Algae typically responds to treatment faster than moss or lichen — visible improvement can occur relatively quickly after treatment.
Call or text: 250-889-8490
Home / The SoftWash Report / Black Roof Streaks
Soft Washing Victoria BC | Roof Moss Removal Victoria BC | House Washing Victoria BC | Pressure Washing Victoria BC | Service Areas | Contact