Black staining on outside walls: why it happens and how to stop it

Almost every wall we are asked to look at because of black or green staining has the same underlying cause: the surface is staying damp long enough for algae and mould to colonise it. Cleaning treats the symptom; keeping the wall dry treats the cause.

Common causes

Painted or sand-and-cement render that has lost its water repellency, north-facing walls that never get sun, overhanging trees and hedges that keep an elevation shaded and still, blocked or leaking gutters soaking one strip of wall, and missing or undersized drip details at sills and verges that let water run back across the face.

Why modern silicone render helps

Silicone thin-coat render is both water-repellent and breathable, so rain beads off the surface rather than soaking in, and any moisture in the wall can still escape. Many systems also carry a biocide in the topcoat. Combined with correct bead and drip detailing, this keeps the wall drying quickly after rain, which is what starves the growth.

The practical fix

Sort the gutters and any obvious water sources first, cut back planting where you can, then either apply a breathable water-repellent treatment to a sound existing render or, if the render is failing anyway, replace it with a modern silicone system. A survey will tell you which of those you actually need.

More articles

• Brick slips vs traditional brick: which is right for your project?
• Do you need planning permission for external wall insulation?
• Insulation grants in the UK: ECO4 and the Great British Insulation Scheme