How to Clean Render Without Wrecking It (Soft Washing Explained)

The short version: don't reach for the jet washer. Modern render is only a few millimetres thick, and high pressure strips it, thins it and forces water into the wall behind. The right way to clean render is soft washing, which means a low-pressure application of a biocide, a dwell time, and a gentle rinse. It kills the algae at the root instead of blasting the surface off along with it.

If your walls have gone green, streaky or patchy on one side of the house, here's what's actually happening and what to do about it.

Why render goes green in the first place

Almost all render staining is organic growth. Algae, mould, lichen and moss, feeding on moisture and airborne spores.

That's why it shows up where you'd expect. North-facing elevations that never get sun. Walls under overhanging trees. The bottom metre where rain splashes back off the ground. Around leaking gutters and downpipes, which is the one to actually worry about, because a green streak below a joint is usually telling you about a plumbing fault rather than a cleaning problem.

Fix the leak first. Otherwise you're cleaning the same stripe again next year.

The other common one is a fine grey film rather than green. That's atmospheric dirt, and in London it's traffic soot as much as anything. It comes off differently and sometimes needs a detergent stage before the biocide.

What soft washing actually is

Soft washing is a chemical process, not a pressure process. That's the whole idea.

A biocide solution goes on at low pressure, no more force than a garden sprayer. It's left to dwell so it can kill the growth down to the root. Then it's rinsed gently, and in many cases the wall keeps getting cleaner over the following days as the dead growth weathers off.

Two things follow from that. First, the wall does not need to be blasted, so the surface is never at risk. Second, because the spores are killed rather than knocked off, it stays clean for years instead of months. Jet washing does the opposite: it removes what you can see and leaves what you can't, so the green is back by the following spring.

You'll sometimes see it called softwashing, low-pressure cleaning or a biocide treatment. Same thing.

Why a pressure washer is the wrong tool

This isn't a cleaner's opinion, it's the render manufacturer's own instruction.

K Rend, whose product is on a very large share of UK homes, states in its own maintenance guidance that a washer "should be set on fan application, not high power (very high pressure washers can damage the render surface)", and that you should clean "in a controlled fashion, working from top to bottom".

Here's what goes wrong when that's ignored:

• You take the surface off. Thin-coat render is measured in millimetres. Blast it and you lose the textured face, leaving patches that never match again.
• You blow the render. Water driven behind the coat causes it to lose its bond, and blown sections eventually crack and fall away.
• You drive water into the wall. On older properties that means damp inside, sometimes months later, and no obvious cause.
• You wreck the seals. Around windows and doors, high pressure finds every weak point in the mastic.

Repairing render costs several times what cleaning it does, and a patch repair almost never colour-matches. That's the real reason we won't put a lance near it.

Which renders need the most care

• Thin-coat silicone and acrylic renders, including K Rend and similar systems. Thin, textured, unforgiving. Soft wash only.
• Monocouche. Through-coloured, so damage shows immediately and permanently.
• Traditional sand and cement render. Tougher, but often older, and frequently hiding cracks that will happily take on water.
• Pebbledash. Looks indestructible. Isn't. High pressure knocks the aggregate out and you end up with bald patches.
• Painted render. Pressure lifts the paint film, and once it starts peeling you're committed to repainting the whole elevation.

Cladding, uPVC, brick and stone all behave differently again, which is why we survey before quoting rather than pricing off a postcode.

What render cleaning costs

Nobody can give you an accurate figure without knowing the height, the elevation count and how bad the growth is. Published UK cost guides put render cleaning somewhere around £5 to £8 per square metre, which works out at roughly £300 to £850 for a typical house, with heavily contaminated or large detached properties going above that.

What actually moves the price:

• Total wall area and how many elevations need doing, since one green side is a much smaller job than all four
• Height and access, whether it can be reached from the ground with a lance or needs a tower
• How established the growth is, because thick lichen needs a longer dwell and sometimes a second visit
• Whether gutters, soffits, fascias or windows are being done at the same time, which is usually cheaper bundled
• Obstructions: conservatories, planting, decking, and neighbours' cars

We quote per property after looking at it. There are no fixed prices on our site for exactly this reason, and anyone giving you a firm number for render cleaning without seeing the walls is guessing.

Can you do it yourself?

Some of it, carefully, at ground level.

What makes render cleaning a professional job isn't the chemistry, it's the height. Most of the stained area on a two-storey house is above ladder height, and the Work at Height Regulations 2005 require work at height to be properly planned, supervised and carried out by competent people, with the right access equipment. HSE's own advice is to avoid working at height where it's reasonably practicable to do so, which is why the trade uses telescopic lances from the ground rather than standing on a ladder with a spray gun.

The other thing to know: biocides are regulated products. They need the right dilution, the right dwell, and proper protection for planting, ponds, pets and neighbouring cars. A strong shop-bought mix used wrongly kills a hedge quicker than it kills algae.

If you do tackle a low wall yourself, test a discreet patch first, keep the pressure low, work top to bottom, and rinse plants before and after.

One more for right now: several London and Hertfordshire suppliers have hosepipe restrictions in force this summer, and they cover jet washing walls at home. Businesses providing the service to customers are treated differently. There's more on that in our guide to cleaning a patio during a hosepipe ban.

How long does it stay clean?

With a proper biocide treatment, typically three to five years before it needs doing again, depending on the aspect and how shaded the wall is. North-facing walls under trees go first.

Jet washed render, by comparison, often looks green again within a single season, because the spores were never killed.

FAQ

Can you jet wash render?
You can put a pressure washer near render on a wide fan at low power, which is what the manufacturer allows for. What you must not do is use a high-pressure jet or a turbo nozzle. The risk of stripping the surface or forcing water into the wall is real and the repair is expensive.

Will soft washing kill my plants?
Not if it's done properly. Planting gets wet down before and after, and anything sensitive gets sheeted. This is the part DIY attempts usually get wrong.

How long does the job take?
Most semi-detached and terraced houses are a day. Larger detached properties or heavy lichen can run to two, or a return visit once the growth has died back.

Does the render need painting afterwards?
Usually not. Cleaning restores the original finish. Painting is only needed if the render was already painted and the coating has failed.

Do you clean gutters and windows at the same time?
Yes, and it's the sensible order. Gutters and fascias first, then walls, then glass. Our professional window cleaners can pick up the glass on the same visit.

Which areas do you cover?
Borehamwood, Elstree, Radlett, Barnet, Watford, St Albans and the rest of North London and the Herts borders. See areas we cover.

Getting a quote

We soft wash render, cladding and exterior walls across North London and Hertfordshire, fully insured with £5 million public liability cover, and rated 4.9 from 180+ reviews.

Send us a couple of photos of the worst elevation and we'll tell you what's growing on it and what it takes to remove. Book a soft washing survey, call 07375 104665, or message us on WhatsApp at wa.me/447375104665.