Short answer
Render is best cleaned at low pressure: apply a cleaner or biocide treatment suitable for your type of render, leave it to work as the label directs, then rinse gently. Jet washing render is risky because high pressure can damage the surface, strip paint and force water into cracks, so it is generally avoided.
Know what kind of render you have
Render is a coating applied to external walls, and there are several kinds. Traditional sand and cement render is often painted. Modern through-coloured renders, including silicone and monocouche types, have colour throughout and are designed to be left unpainted. Some have a textured or pebble-dash finish.
The type matters because each reacts differently to cleaning products and pressure. Silicone and other modern systems often come with manufacturer cleaning guidance, which may also affect any guarantee. If you have paperwork from when the render was applied, check it before you start. If not, a render installer or cleaning professional can usually tell you what you have.
Why render goes green and black
Green staining is usually algae, which thrives on walls that stay damp and see little sun, especially north-facing elevations and walls near trees or hedges. Black streaks often run down from sills, ledges and the corners of windows where rainwater carries dirt across the same path, and dark spotting can also be mould or lichen.
Algae and mould are living growth. Rinsing them off the surface without killing the spores underneath tends to bring a short-lived result, which is why biocide treatment is part of most render cleaning.
What is biocide treatment?
A biocide treatment is a solution that kills algae, mould and lichen on the surface. It is applied at low pressure, left to work for the time the product specifies, then rinsed or left, depending on the product. Because it kills the growth rather than just washing it, the wall usually stays cleaner for longer, and faint staining can continue to fade after treatment.
Our guide to soft washing vs pressure washing explains how this low-pressure, solution-based approach compares with jet washing on other surfaces.
How to clean a small area of render
- Choose a dry, overcast day, so the product does not dry on in hot sun or wash off in rain.
- Cover or soak nearby plants with plain water, and protect windows, doors and anything that could be marked by run-off.
- Choose a render cleaner or biocide suitable for your render type and read the instructions fully. Never mix products.
- Test it on a small, hidden patch and let it dry before judging the result.
- Apply the product at low pressure, for example with a garden sprayer, working from the bottom of the wall upwards so it does not run in streaks over dry render.
- Leave it for the contact time the label gives. Avoid scrubbing textured or through-coloured render hard, which can mark the surface.
- Rinse gently with a hose if the product requires it, working from the top down.
Can you jet wash render?
It is generally best not to. A high-pressure jet can erode the surface of render, leave visible lines, blow off paint and push water into hairline cracks, where it can cause problems later. On through-coloured renders, damage to the surface can leave the colour patchy in a way that cannot simply be painted over.
Some render manufacturers allow low-pressure rinsing within certain limits, so check the guidance for your system. If in doubt, use the gentle method above or leave it to a professional.
Safety first
Only clean the parts of a wall you can reach safely from the ground. Wear gloves and eye protection when handling cleaning solutions, keep children and pets away while you work, and follow the product's safety advice. Upper storeys are a job for someone with the right equipment and training.
When to call a professional
Call a professional if the staining covers whole elevations, reaches above the ground floor, or keeps coming back after you clean it. It is also worth getting advice if the render is cracked, flaking or patchy, because cleaning will not repair it, and the wrong product can make marks worse.
We soft wash render using low pressure and a specialist solution that kills algae at the root, and we avoid pressure washing render because it can damage it. Silicone, monocouche and painted render are all treated this way, with windows, plants and nearby surfaces protected while we work.
Rather have it done for you?
Soft washing of render, brick and stone to remove algae and black streaks. Family-run by Jason and Lucas Prior, covering south Essex.
Frequently asked questions
Can you pressure wash render?
It is generally avoided. High pressure can damage the render surface, strip paint and drive water into cracks. A low-pressure clean with a suitable biocide is the safer approach.
What is the best thing to clean render with?
A cleaner or biocide treatment made for render and suitable for your render type, applied at low pressure and used exactly as the label directs. Test a hidden patch first.
How long does render stay clean after treatment?
It varies with the wall, the shade and the surroundings. Killing the growth rather than just rinsing it usually keeps a wall cleaner for longer, but shaded, damp walls tend to need attention sooner.
Related guides
- Soft washing vs pressure washing: what soft washing is, how it differs from pressure washing and which outdoor surfaces suit each method.