Black Streaks (Algae)
Gloeocapsa Magma AKA ‘Black Streaks – Algae’ Explained
A Summary in Brief Those dark streaks running down your roof are more than just stains. In most Northern Virginia neighborhoods, they are caused by Gloeocapsa magma, a roof-eating bacterium that feeds on the protective granules in your shingles. Over time, it makes your home look neglected, can trigger HOA notices, and can shorten the…
Read MoreWhy Black Streaks Appear on Your Sidings and Roofs
Published November 18, 2025 – Last reviewed February 21, 2026 Michael Thorgenson Owner, Aqua Clean Solutions, Inc. Introduction: In a Nutshell Black streaks on roofs in Northern Virginia are primarily caused by Gloeocapsa magma, a cyanobacteria (blue-green algae-like organism) that feeds on the limestone filler in asphalt shingles, forming protective dark pigmented sheaths that create…
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