Why Is My Render Going Green – and What It’s Doing to the Wall

A rendered wall going green is one of the most common things we get called about, and it is almost always the same story: the front of the house looks fine, one gable or the back has turned, and it seems to have happened quickly. It did not happen quickly. It reached a threshold where you could suddenly see it.

The short answer: algae, feeding on damp

The green is algae. It needs very little to establish itself on a wall: moisture, somewhere to sit, and light. In Ireland the moisture is rarely the limiting factor, so what usually decides whether a wall goes green is how long that wall stays wet after it rains.

That is why this is not really a cleanliness problem. A green wall is telling you it is holding moisture, and that is worth understanding before you decide what to do about it.

Why one elevation goes first

North-facing walls get the least direct sun, so they dry slowest and stay damp longest. Shade does the same job: a wall behind a hedge, under trees, beside a boundary wall or in the lee of an extension will hold moisture far longer than an open, south-facing elevation on the same house.

Other things quietly make it worse. A blocked or leaking gutter above a wall keeps a strip of it permanently wet. A downpipe discharging badly does the same. Splash-back from a hard surface at the base keeps the bottom metre damp. If one patch of a wall is far worse than the rest, look up before you look at the render.

A green wall is not dirty. It is damp, and the green is what grows on damp.

What it is doing to the wall, beyond the look

Render is meant to get wet and then dry. Algae interferes with the drying half. A living layer across the surface holds water against the render and keeps it damp for longer after every shower, so the wall spends far more of the year wet than it was ever intended to.

That matters for the same reason it matters on a roof. A surface that stays saturated goes through every freeze and thaw of the winter holding water, and that cycle works away at the finish over time. Persistent damp also tends to find whatever weakness already exists, so hairline cracks, poor detailing and failed seals get worse faster than they otherwise would.

None of this is sudden or dramatic. It is the difference between a finish ageing at the rate it should and ageing faster than it should.

Silicone and through-coloured renders

Modern through-coloured silicone renders, K Rend among them, are popular here for good reason and they are not unusually prone to this. What they are is unusually good at showing it.

The colour is consistent across the whole surface, so any patchiness reads immediately, and light or neutral colours show green far sooner than a painted wall that has already weathered unevenly. Owners often conclude the render has failed. Usually it has not; it is doing exactly what it was meant to and simply making the growth obvious earlier.

The practical consequence is that these finishes should be cleaned in a way that respects the surface, because the finish is the material itself rather than a coat of paint over it. Damage the face and you cannot simply paint over the result.

Why jet washing makes it come back faster

This is the expensive mistake. A pressure washer will take the green off a wall and the immediate result looks convincing, which is exactly why people do it.

Two things go wrong. Pressure abrades the render surface, and a roughened, more open surface holds moisture more readily than a sound one, so the wall becomes better habitat than it was before. And blasting the growth off removes what you can see while leaving the wall no less hospitable, so it re-establishes from what remains.

The pattern is that the second growth arrives sooner than the first, and the wall looks worse each time. That is not bad luck; it is the method.

What actually works

Softwashing takes the opposite approach. Instead of removing growth by force, a treatment is applied at low pressure that kills it, and the wall clears as the dead material weathers off. The render surface is not abraded, so it is not left more absorbent than it started.

It also treats the whole elevation rather than the visible patches, which matters because the areas that have not obviously turned yet are usually on the same path a little further behind. That is the same reasoning as roof treatment, explained in why biocide matters.

If you want the difference between the two methods surface by surface, see soft washing vs pressure washing.

Common questions

Related services

• Softwashing for render and walls
• Gutter cleaning
• Roof cleaning