Why Is My Render Going Green?

The direct answer: it's algae, not a render fault

Green staining on render is caused by microscopic algae spores that are constantly present in the air, landing on the wall surface and establishing wherever conditions are damp and shaded enough to support growth. Render's slightly textured, porous surface (particularly common finishes like K-Rend, pebbledash and monocouche) gives these spores plenty to grip onto, far more than a smooth painted brick wall would.

It's important to know this isn't a sign of poor-quality render or installation, it's simply biology finding a suitable surface, though certain render types and colours do show it more visibly than others, and paler renders make green staining stand out more starkly by contrast.

Why north-facing walls are worst affected

North-facing elevations receive little to no direct sunlight through the day, which means they dry out far more slowly after rain than south or west-facing walls. That extended damp window is exactly what algae needs to establish and spread, which is why it's extremely common to see one wall of a house significantly greener than the rest, even though every wall was rendered at the same time with the same product.

Overhanging trees, nearby fences or walls that cast shade, and poor airflow around a property (narrow side passages, for example) all compound the effect in the same way, adding to the time render spends damp between weather events.

A realistic prevention plan

A professional soft wash, low-pressure application of a biocide treatment that kills algae spores at the surface, followed by a gentle rinse, removes existing green staining without damaging the render's texture or colour, which pressure washing frequently does by eroding the render's surface coat.

Prevention afterwards is realistic rather than permanent: an annual or biennial soft wash on the worst-affected elevations, timed before spring growth really gets going, keeps most homes looking clean year-round rather than letting staining build back up to the point it needs a full intensive clean again.

It's also worth having a quick look at anything nearby that might be adding extra shade or moisture, overgrown shrubs planted close to the wall, a leaking gutter overhead, or a downpipe discharging directly onto the render, since dealing with those alongside the clean often extends how long the result lasts.