Why Is My Render Going Green? Causes, Fixes and Prevention

What is the green on my render?

That green tinge creeping across your walls is almost always algae. It is a simple plant-like growth that loves damp surfaces and spreads easily across render, especially the textured and silicone renders used on many modern homes.

You may also see black spots or streaks, which are often mould or a darker type of algae, and small crusty patches of grey, yellow or orange, which are usually lichen. Lichen grips on tightly and is slower to shift.

None of these mean your render has failed, but left alone they will keep spreading and can make a well-kept house look tired and neglected.

Why some walls go green faster than others

Algae needs moisture and shade, so north-facing walls and walls that don't get much sun are usually the first to turn green. Walls near trees, hedges, fences or neighbouring buildings stay damp for longer after rain, which gives growth a head start.

Our damp North East climate doesn't help. We get plenty of rain, mist and grey days round here, and walls that catch the wind-driven rain get it worst. Render in these conditions simply has more chances to stay wet.

It is also common to see growth near the ground, where rain splashes back up the wall, and around bushes or plants that sit close to the house.

Streaks under gutters and windowsills

If you have green or dark streaks running down from a gutter, downpipe or windowsill, the cause is usually water. A blocked or leaking gutter, a dripping joint or a sill that doesn't throw water clear of the wall will send a steady trickle down the same path every time it rains.

Cleaning the render will remove the staining, but if the water keeps running down the wall the marks are likely to come back sooner. It is well worth having gutters cleared and any leaks fixed at the same time.

Why washing alone isn't enough

It can be tempting to hire a pressure washer and blast the green off. It might look better for a while, but it only removes what you can see.

The roots of algae and lichen are often left behind in the render, so the growth can come back quickly. For render, treating the growth is what makes the clean last.

What is soft washing?

Soft washing is the method we use for render. Instead of high pressure, we apply specialist cleaning solutions at low pressure. These are designed to break down and kill algae, mould and lichen at the root, rather than just knocking the surface off.

Once the solution has had time to work, we come back about 6 weeks later to treat any remaining staining and rinse the walls off with light pressure. Mark is a City & Guilds qualified soft wash technician and a member of the Exterior Cleaning Association, so the right products are used in the right way for your type of render.

It isn’t a one-visit job: we come back about 6 weeks later to treat any remaining staining and oxidation, as stubborn marks like lichen take longer to shift.

How long will it stay clean?

Because soft washing treats growth at the root, render usually stays clean for much longer than it would after a simple wash. How long depends on your property: a shady, north-facing wall near trees will naturally turn green sooner than a sunny, open one.

To help it last, keep gutters clear, fix any leaks or drips, and trim back plants and hedges that sit against the walls. Many homeowners choose a periodic clean to keep on top of things before the green returns.

Thinking of painting your render?

If you are thinking of painting your render, it is important to clean it properly first. Paint applied over algae or mould is unlikely to bond well, and growth can show through or return underneath. Soft washing gives your decorator a clean surface to work on.

Often, though, people find that a good clean brings the render back to such a fresh colour that they no longer need to paint at all. Render cleaning starts from £250. For a free quote, call 07584 956981.