Why render goes green, black or red — and how it should be cleaned
Render changes colour because something is growing on it, or because something is running down it. The colour tells you which, and that determines the method. Cleaning the wrong way on render is one of the few exterior mistakes that cannot be undone.
Green
Algae. It is the most common and the least serious. Green appears first on north and east elevations, under overhanging trees, and on the shaded half of a building, because those surfaces stay damp longest. On K-Rend and other silicone renders it looks worse than it is: the render is designed to shed water, so the growth sits proud on the surface rather than soaking in.
Treatment: soft wash with a biocide and a long dwell, then a gentle rinse. Not a pressure washer, ever, on silicone render.
Black
Usually a mix of algae and airborne particulate that has bonded to it, and on older surfaces, lichen. In city locations you also get carbon deposit from traffic, which is not organic and does not respond to a biocide alone. Black staining under a window sill or below a coping stone is a drainage detail, not a cleaning problem: water is running down the face rather than dripping clear.
Treatment: biocide for the organic part; where there is bonded particulate a second stage is needed. Where the cause is water tracking off a sill or coping, cleaning it without fixing the detail buys you a year.
Red or orange
Two different causes and they need telling apart.
- Red algae (often Trentepohlia) is a genuine biological growth that reads as rust-orange. It is common on rendered walls and roof tiles in the West Country and responds to biocide like any other algae, though it usually needs a longer dwell.
- Iron staining is not biological. It runs from railings, gate posts, planters, fixings or the substrate itself, and it will laugh at a biocide. It needs a specific iron-removal treatment, applied carefully, because the same chemistry that lifts the stain can etch what is underneath.
The practical test is direction. Growth appears in patches and on damp elevations. Iron runs in streaks from a point, and there is usually something metal above it.
Pink or purple
Less common, and usually a different algal species or the early stage of lichen. Same treatment family as green, longer dwell.
What decides the method, not the colour
What the render is made of matters more than what is growing on it.
- Silicone render (K-Rend and similar): soft wash only. It marks permanently under pressure.
- Painted render: soft wash only. Pressure lifts the paint film and you repaint the elevation.
- Pebbledash: soft wash only. Pressure strips the aggregate and it does not go back.
- Cement render, unpainted: tolerates more, but cleans unevenly if it is done in patches.
- Lime render: softer than the stone around it. Low pressure, long dwell, and on a listed building check whether consent is needed first.
What happens if it is left
Algae itself is not structurally damaging in the short term, but it holds moisture against the surface, and a render face that stays wet through a winter is a render face that will craze and open sooner. Lichen is more aggressive: it roots into the surface, and by the time it is established, removing it takes some of the render with it. The cheap moment to deal with render is when it is green, not when it is black and lichened.
How long a clean lasts
Three to five years on an average elevation, less on a permanently shaded north face under trees, more on an open south-facing wall. A residual biocide after the wash is what moves the number, not how hard the water hit.
Frequently asked questions
Why does only one side of my house go green?
Sunlight and drying time. North and east elevations stay damp longer, which is the condition algae needs. It is normal and it is not a sign of a fault in the render.
Is the orange staining on my render rust or algae?
Look at the shape. Algae appears in patches on damp areas. Iron runs in streaks from a fixing, railing or planter above. They need completely different treatments, so it is worth identifying before anyone starts.
Will cleaning damage my K-Rend?
Not with a soft wash. It will with a pressure washer or a rotary head, and that damage is permanent.
Can you clean render on a listed building?
Often yes, at low pressure with a long dwell, but cleaning a listed elevation can need Listed Building Consent from the council. We raise that at quote stage rather than after work has started.
Related
Render & wall cleaning · K-Rend & silicone render cleaning · Soft washing · Soft washing vs pressure washing