In short
Green on render is algae, and it grows where the wall stays damp: north-facing elevations, walls shaded by trees or hedges, and anywhere a leaking gutter keeps the surface wet. It does not damage the render itself, but it looks poor and it tells you that part of the wall is holding water. A gentle wash with a biocide clears it; fixing the damp source stops it coming back.
This is a job for our Rendering service.
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Why it grows where it grows
Algae spores are in the air everywhere. They only take hold on a surface that stays wet long enough. On the jobs we see across South Norfolk and the Waveney valley, the green wall is nearly always the north or east elevation, the one facing the paddock hedge, or the bit under a dripping gutter. K Rend's own maintenance guidance says the same: algae grows on damp surfaces, and worst near woodland.
Painted sand and cement render is the most affected, because the surface soaks up water and dries slowly. Old pebbledash is nearly as bad. Silicone render behaves differently. K Rend's silicone finish cuts water absorption at the surface while letting vapour out of the wall, so it dries quickly after rain and, in their words, is drier and therefore more resistant to algae growth.
Does it damage the wall?
Not directly. Algae sits on the surface. What matters is what it tells you: that part of the wall is staying wet. Wet render freezes in winter and cracks, and a wet wall loses heat. So treat the green as a warning sign and look for the cause: a blocked gutter, a broken downpipe, a hedge tight against the wall, or a render that simply holds water.
How to clean it off
- Apply a biocide or algaecide and let it work. K Rend recommend their Algae Clean product for this; other manufacturers have equivalents.
- Wash down from the top with clean water and a mild detergent. K Rend say a pressure washer on a fan setting is fine; a narrow jet at close range will scar the surface and take colour off.
- Let it dry and look again. Stubborn staining on old cement render may need a second treatment.
K Rend describe an annual gentle wash, the same as you would give a patio, as all a silicone finish needs. That is our experience on the houses we have rendered too.
Stopping it coming back
- Fix the water source first. Most often that is guttering, and we do that work ourselves.
- Cut hedges and trees back so air and sun reach the wall.
- On a silicone finish, a protective coating such as K Rend's K Pro increases water repellency and keeps the wall cleaner for longer without sealing it up.
- If the wall is old painted cement render that greens up every year, re-rendering in a through-coloured silicone system deals with it for good and gives you a wall that no longer needs painting.