What Is Algae Staining & Black Spot Discoloration?
Algae discoloration forms as single-celled or colonial algae colonize exterior surfaces. Unlike moss, algae creates a slimy, staining growth rather than thick carpet growth. Black algae (most common in UK) appears as dark staining that makes surfaces look dirty and aged.
Causes & Triggers
Algae thrives in damp conditions with regular moisture exposure. Wind-driven rain on north-facing surfaces, spray from gutters, and poor drainage all encourage algae growth. Shaded areas with reduced sunlight create ideal conditions. Gutters, roof tiles, render, and patios are particularly vulnerable.
When Is It Worst?
Algae grows year-round but appears most visible in spring and summer when moisture levels peak.