Render Cleaning: How to Clean Render Without Ruining It

Why pressure washing ruins render

A pressure washer strips the outer layer of any render it is used on. On through-coloured silicone systems such as K-Rend, that outer layer carries the colour and the water-repellent property, so removing it leaves permanent patchy lightening and a surface that now absorbs water. On painted render, the paint film comes off in strips. On pebbledash, the aggregate is knocked out.

Worse, high-pressure water finds every hairline crack and drives moisture behind the render. Trapped water then freezes, expands and blows the render off the substrate in sheets, which is a repair running into thousands rather than a cleaning bill in the hundreds.

The correct method: soft washing

Render should be soft washed. A dilute biocidal solution is applied at low pressure, allowed to dwell so it kills the algae, fungal film and lichen colonising the surface, and then either rinsed at very low pressure or left to weather clean. Nothing is blasted, so the surface finish and any water-repellent coating remain intact.

As with a roof, the visual result develops rather than appearing instantly. Green algae typically fades within days; heavier black staining and lichen take weeks. A second lighter application is sometimes needed on badly affected north-facing walls, and that is normal rather than a sign of failure.

• Sheet or wet down planting at the base of the wall first
• Protect windows, doors and any timber
• Apply solution at low pressure, working bottom to top
• Allow proper dwell time
• Rinse gently or allow natural weathering

Different render types, different care

Silicone and acrylic through-coloured renders are the most vulnerable to pressure and the most rewarding to soft wash, since the colour beneath the growth is genuinely intact. Traditional sand-and-cement render, especially when painted with masonry paint, tolerates a little more but still should not be pressure washed if the paint film is to survive.

Pebbledash and roughcast hold dirt in their texture and often look dramatically better after treatment, but the aggregate is only mechanically keyed in — pressure removes it. Lime render on period properties needs the gentlest handling of all and specialist advice, as it is deliberately soft and vapour-open.

Dealing with the causes, not just the symptoms

Render goes green because it stays wet. Before cleaning, look at why. Overflowing gutters, a blocked downpipe, a missing drip detail under a sill, or splash-back from a path or patio at the base of the wall will all keep a section permanently damp and recolonised within a year.

Fixing the cause is what makes the clean last. A wall cleaned while a leaking gutter above it is still leaking will show a green stripe again by the following spring.

Cost and frequency

Soft washing render on a typical three-bed semi generally costs £350 to £700 depending on height, access and the number of elevations. Single elevations are often £150 to £300. Detached and larger properties scale upwards from there, and scaffolding or MEWP hire where needed is a separate significant cost.

Most rendered properties benefit from cleaning every three to five years. Shaded elevations under trees may want a lighter maintenance treatment more often, which is much cheaper than allowing heavy growth to re-establish.

The bottom line

Render is the surface where choosing the wrong contractor is most expensive, because the damage is instant, permanent and highly visible. A pressure-washed silicone render cannot be restored — it has to be recoated or re-rendered, which is a job in the thousands. Before anyone touches a rendered wall, get written confirmation that the method is a low-pressure soft wash.

Once it is clean, spend ten minutes finding out why it went green in the first place. A leaking gutter joint, a blocked downpipe or a missing drip detail under a sill will bring the staining back in a single season, and fixing it is usually a job of minutes. Cleaning treats the symptom; the small repair above it is what makes the result last five years instead of one.

Frequently asked questions