Black Staining on Render: Causes and Safe Cleaning

By Mark Cave October 09, 2026 |

Why Black Staining Appears on Render Black staining on render is usually caused by biological growth, most commonly algae, cyanobacteria, mould, mildew and other airborne microorganisms that colonise damp exterior surfaces. On UK properties, it is especially common on shaded, north-facing or poorly ventilated elevations where render stays wet for...