How to get green algae off render and cladding

The green on render and cladding is algae, and the way to get it off without damaging the wall is a soft wash: low pressure, a soft brush and plenty of water, working from the top down. High pressure takes the algae off but can also take the surface off with it, drive water behind cladding and leave a patchy, striped finish.

Here is why walls go green, which ones go first, and how to clean them so they stay clean for longer.

Why walls go green

Algae needs moisture and light, and not too much sun. A wall that stays damp for long periods and does not dry out quickly is ideal for it. That is why you see it most on:

• North-facing walls that rarely get direct sun
• Walls overshadowed by trees, hedges or the house next door
• Areas under gutters that overflow, or below window sills that drip
• Coastal properties, where damp sea air keeps surfaces wet
• Textured or rough render, which holds water and dirt in its surface

In South Devon the mild, wet winters mean algae can keep growing for much of the year. Plenty of homes in Torquay, Paignton and Brixham have one elevation that is spotless and another that is green, simply because of which way they face.

Black streaks under sills

The dark streaks that run down from the corners of window sills are a slightly different problem. Dirt and traffic film settle on the sill, rain washes it over the edge in the same spot every time, and it leaves a streak that builds up over the months. Algae often grows in the same line because the wall stays wet there. Both come off with the same soft wash, and cleaning the sills at the same time slows them coming back.

Why pressure washing is the wrong tool

A pressure washer looks like the quick answer, and on a concrete drive it often is. On a wall it causes problems:

• It can erode the surface of render and strip painted finishes, leaving a lighter patch where the lance was held
• It forces water into hairline cracks and behind cladding boards, where it can sit and cause damp
• It tends to leave stripes and tide marks, because the clean area depends on how long the lance stayed in each spot
• It can blow apart weak mortar on older brick and stone

Soft washing uses low pressure and lets the water and brush do the work. It is slower, but the finish is even and nothing is forced where it should not be.

How a soft wash works

1. The wall is checked first for cracks, loose render and areas that need a gentler approach.
2. Surrounding plants and paving are wet down so run-off does not mark them.
3. The wall is washed from the top down with a soft brush on a pole, so dirty water runs over areas still to be cleaned rather than onto clean ones.
4. Fascias, soffits, downpipes and window frames on the same elevation are washed at the same time so nothing is left looking dirty next to clean render.
5. Everything is rinsed with clean water.

Because it is all done from the ground with a pole, there is no scaffold and nothing leaning against the wall.

Can you do it yourself?

On a low garden wall or a single storey extension, a soft brush, a hose and patience will shift light green growth. Work from the top, keep the brush soft, and do not be tempted to turn up the pressure on the stubborn bits. Anything above head height, or a whole elevation of a house, is easier and safer with the right kit.

Stopping it coming back

• Fix the source of water. An overflowing gutter will turn the wall below green again quickly, so check the gutters first. Our guide to the signs your gutters are blocked explains what to look for.
• Cut back trees and shrubs that shade the wall or touch it.
• Keep sills and frames clean, which slows the black streaking.
• Plan a wash every one to two years. Shaded and north-facing walls may need doing on their own in between.

Render and cladding cleaning in South Devon

AbraClean soft washes render, UPVC and composite cladding across Torbay, South Devon and Exeter. See our cladding and render cleaning service, and consider adding gutter clearing and window cleaning at the same visit. Areas include Torquay, Brixham and Paignton. Call or text Mark on 07901 563000 for a free quote.