Why is my render going green? Causes and how to fix it

Updated 6 November 2026 · By the Render Cleaning team

Short answer: Render goes green because algae grows on surfaces that stay damp, especially north-facing walls, walls near trees and areas below sills and copings. Black marks are usually mould or other algae, and orange staining is often Trentepohlia algae or iron in run-off. The fix is a low-pressure clean with a suitable biocide, ideally followed by a protective treatment.

Common causes

North-facing walls that get little sun and dry out slowly
Overhanging trees and hedges that drip and shade the wall
Water running off sills, copings and gutters
Rural and coastal locations with more airborne spores
Leaking or overflowing gutters and downpipes

What the colours mean

Colour	Usual cause
Green	Algae on damp, shaded walls
Black or dark grey	Mould, black algae or atmospheric dirt
Orange or red	Trentepohlia algae or iron in run-off water
White patches	Efflorescence (salts) or lichen

Does silicone render go green?

Yes. Silicone render repels water better than traditional render, so it tends to stay cleaner for longer, but algae can still grow on it, especially on shaded walls. Through-coloured monocouche render can be affected in the same way.

How to fix it and slow it returning

Clean at low pressure with a biocide suited to the render type
Avoid pressure washing, which can damage the surface
Fix leaking gutters and overflowing downpipes
Cut back trees and shrubs touching or overhanging the wall
Apply a protective treatment and plan a light maintenance clean every year or two