In short
- Green, red and black staining on render is usually algae, lichen or mould on shaded, damp elevations.
- The safe method is a biocidal soft wash at low pressure, left to work, then rinsed. High-pressure jet washing can damage thin-coat render.
- Silicone and monocouche usually clean well; recolouring with a silicone paint is an option where cleaning is not enough.
- Tree cover, north-facing walls and dripping sills are the main local causes. Fixing drips helps prevent regrowth.
Why render goes green in Surrey
Much of Surrey is wooded, and many houses around Guildford, Cranleigh, Haslemere and Cobham sit under mature trees. Shade plus damp plus a slightly textured surface is ideal for algae. North-facing elevations dry slowly, and walls below sills without projecting drips get concentrated run-off. Modern renders contain biocides, but these leach out over time, which is why a render that stayed clean for its first few years can start to colour.
How render should be cleaned
- Protect plants, glazing and paintwork.
- Apply a biocidal cleaning solution suited to the render type, at low pressure.
- Allow dwell time for the biocide to kill the growth; some products continue working for weeks.
- Rinse at low pressure, working from the top down.
- Where appropriate, apply a protective treatment to slow regrowth.
High-pressure jet washing is fast, but it can erode the surface of thin-coat renders, drive water into cracks and leave a patchy result. We always clean at low pressure with a biocide, never with a high-pressure jet.
When cleaning is not enough
Very old or heavily weathered render may clean up unevenly. Options are a silicone-based facade paint in the original or a new colour, or a thin-coat re-render over the existing surface if it is sound. See cleaning silicone render.
Preventing regrowth
- Cut back overhanging branches where you can.
- Fix leaking gutters and downpipes.
- Fit or extend sills with proper drips.
- Plan a gentle clean every few years rather than waiting for heavy growth.
Questions people ask
Can I pressure wash my render?
Not at high pressure on thin-coat render. Low-pressure washing with a suitable biocide is safer and more effective.
How often does render need cleaning?
It varies with shade and exposure. Many silicone renders go several years between cleans; shaded, tree-lined walls need it more often.
Can stained render be recoloured?
Yes. Silicone facade paints can refresh or change the colour of sound render while keeping it breathable.