Advice · 4 min read · Updated 6 November 2026
Render cleaning myths: what really works and what damages walls
Myth: pressure washing gets render cleanest
A pressure washer gives a dramatic instant result, which is why the myth persists. But on most renders, high pressure removes some of the surface itself along with the dirt. On silicone and monocouche, it can strip the water-repellent surface or texture. Spores left deeper in the surface regrow quickly, often faster than before because the surface is now rougher.
Low pressure soft washing with a biocide kills growth in the surface without blasting it, and usually lasts longer.
Myth: silicone render never needs cleaning
Silicone renders are marketed as self-cleaning or dirt-resistant, and they do shed dirt and water better than many older renders. But they are not maintenance free. Shaded and north facing walls, walls near trees and walls in damp areas can still go green or black within a few years, especially as the water-repellent effect slowly weakens over time.
| Claim | Reality |
|---|---|
| Self-cleaning | Sheds dirt better, but still collects algae in damp shaded spots |
| Never goes green | Can go green, especially on north walls |
| Any cleaner is fine | Harsh methods can strip the water-repellent surface |
| Lasts forever | Water repellency reduces gradually, treatments can help |
Myth: bleach is the best cheap fix
Bleach kills algae on contact, but it can lighten coloured render, leave streaks, harm plants, corrode metal and does not always reach growth below the surface. On through-coloured render, bleach damage can be permanent. Products designed for render are formulated to work without these side effects when used correctly.
Bleach also tends to whiten the visible growth without killing everything rooted in the surface, so a wall can look clean for a few weeks and then green over again. That makes it a false economy, even before any damage to the finish is counted.
Myth: if it looks clean, it is clean
Algae and lichen spores can remain in the surface even after visible staining has gone, particularly after pressure washing. This is why some walls go green again within months. A biocide treatment kills the growth throughout the surface, and a residual treatment can slow it coming back.
Some biocides also work the other way round: the wall may not look perfect straight away, but continues to clear over the following weeks as rain washes away the dead growth.
Myth: painting over stained render solves it
Painting over green or black growth without cleaning and treating it first usually fails. The algae can grow through or under the paint, and paint may not bond properly. On through-coloured renders such as silicone or monocouche, painting can also change how the wall handles moisture. Clean and treat first, let the wall dry, then decide whether painting is needed at all.
Painting also commits you to repainting in future. Once a through-coloured render has been painted, it will need repainting every several years like any painted wall, which is a far bigger and more expensive job than an occasional clean.
Myth: green render means damp inside
Green staining on the outside of render usually means the surface stays damp for long periods, but it does not automatically mean damp inside the house. However, it can be a sign of a related problem such as a leaking gutter, cracked render or a missing sill drip, and those should be checked. If you see damp patches indoors on the same wall, investigate further.
- Outside staining only: often just shade and moisture on the surface
- Staining with cracks: check whether water is getting behind the render
- Staining below a gutter or downpipe: look for leaks or overflow
- Damp inside on the same wall: investigate the cause before cleaning
What actually works
One last myth is that a single clean lasts forever. Render cleaning is maintenance, not a permanent fix. Even the best clean and protective treatment will eventually be overtaken by weather, shade and airborne spores. How long a clean lasts depends on the wall's orientation, nearby trees and fields, local rainfall and the condition of gutters and sills. A few years is common on shaded walls, longer on sunny ones. These steps give the best result for the money:
- Identify the render type before choosing a method
- Use low pressure and a product suitable for that render
- Allow the product time to work, as some act over weeks
- Fix gutter leaks, overflows and missing drips that feed the growth
- Consider a protective treatment in damp or shaded locations
- Check the walls each year and clean before staining becomes heavy
Frequently asked questions
Is soft washing really better than pressure washing for render?
For most renders, yes. Soft washing uses low pressure and a cleaning solution or biocide to kill algae, lichen and moss in the surface, then rinses gently. Pressure washing relies on force, which can strip silicone, monocouche and painted finishes and leave spores behind. Soft washing is generally gentler on the render and gives longer-lasting results.
Why does my silicone render still go green?
Silicone render resists dirt and water better than older renders, but it is not immune to algae. North facing walls, walls near trees or fields, and walls in damp areas stay wet longer, giving algae a chance to grow. The water-repellent effect also weakens gradually with age. Gentle cleaning every few years and a protective treatment can help.
Can I paint over green render?
It is not a good idea to paint over green or black growth without cleaning and treating it first. The algae can grow back through or under the paint and stop it bonding properly. Clean the render with a suitable biocide, let it dry thoroughly, and then decide whether painting is still needed. Through-coloured renders often look fine after cleaning alone.
myths silicone render soft washing maintenance
