In short
- Algae grows once the render's built-in biocides leach out, especially on shaded, damp walls.
- Use a biocidal cleaner at low pressure, allow dwell time, then rinse gently.
- Avoid high-pressure jet washing, which can erode the thin topcoat.
- Fix drips, gutters and overhanging branches to slow regrowth.
Why silicone render stains
Silicone render contains biocides to deter algae and fungi. Over years they slowly wash out. On walls that stay damp, north faces, walls under trees, below sills without drips, spores settle and grow. Green is usually algae, black is often mould or pollution, and orange or red can be a specific algae species.
The right way to clean
- Wet surrounding plants and protect glazing.
- Apply a biocidal render cleaner suited to silicone, at low pressure.
- Leave it to work as directed; some products continue working over days or weeks.
- Rinse gently from the top down.
- Consider a protective treatment afterwards.
What not to do
- Do not use high-pressure jet washers close to thin-coat render.
- Do not use bleach-based products not designed for render; they can discolour.
- Do not scrub with hard brushes.
Recolouring
Where cleaning leaves an uneven result, silicone facade paints can refresh or change the colour while staying breathable. See render cleaning.
Choosing a cleaning product
Use a cleaner sold for masonry and render, ideally recommended by the render manufacturer. Products differ: some act quickly and are rinsed off; others are left on to keep working for weeks as rain activates them. Follow the dilution and dwell times exactly, and test a small area first.
Doing it yourself safely
For ground-floor areas, a garden pump sprayer and a soft brush are enough. Work from the bottom up when applying, so the product does not streak, and rinse from the top down. Do not work from ladders on upper floors with chemicals; a professional with proper access equipment is safer.
Professional cleaning
Professionals typically use low-pressure spray systems from access towers or cherry pickers, with biocide dwell and a gentle rinse. Some offer a protective treatment afterwards. Ask what products they use and whether they are suitable for your render system.
Stains that are not algae
Not every mark is organic. Rust streaks from old fixings, efflorescence from salts, lime bloom on monocouche and staining from failed sealants each need different treatment. If a cleaner does not shift a mark, have it identified before trying anything stronger.
Questions people ask
Can I clean my own render?
Yes, on ground-floor areas with a suitable render cleaner and a garden sprayer. Work at height is best left to professionals.
How often should silicone render be cleaned?
Every few years on shaded walls; less often on sunny, open elevations.