Why Your Roof Has Black Streaks (and What Actually Removes Them)

The black streaking on asphalt roofs across the CSRA is a blue-green algae called Gloeocapsa magma. It thrives in warm, humid climates and feeds on the limestone filler manufacturers mix into shingles.

It spreads by airborne spores, which is why it usually appears on the north-facing slope first, then on your neighbor's roof next.

The dark colonies absorb heat, which raises attic temperatures and accelerates shingle aging. The algae also holds moisture against the roof surface and progressively loosens granules, shortening the service life of the roof.

Realtors and appraisers notice it too — a heavily streaked roof reads as a roof near the end of its life even when it has a decade left.

Pressure washing removes streaks and granules together. Walking a hot roof with a scrub brush cracks shingles. Zinc or copper strips at the ridge help slow future regrowth slightly but do nothing about staining that is already there.

A soft wash with a shingle-safe solution applied at low pressure. It kills the organism outright, and the dead growth releases and rinses off with rainfall over the following weeks. Done properly it is the method shingle manufacturers themselves recommend.

In the CSRA, most roofs need this every three to five years, and sooner if they sit under pines.