Locally owned & family run · On time, on point · Free quotes

Home → Mould on render

House washing

How to get mould off render without wrecking the paint

The black film on a rendered wall is not dirt. It is alive. Kill it and it goes; blast it and it comes back darker. Here is what actually works on a Toowoomba house, and what should never go near your walls.

Get your free quote

What it actually is

That black film is a living colony

What reads as grime on a rendered wall is almost always mould, mildew, algae and lichen growing on the surface and feeding on moisture, pollen and airborne dust. Toowoomba supplies all three generously: fog that sits until mid-morning, an elevation that keeps walls cold and damp well into the day, and a growing season that never properly stops.

That distinction matters, because it makes this a biological problem rather than a dirt problem. Water and scrubbing take off the visible layer while leaving the root structure down in the pores of the render. Two or three months later it is back, usually darker than it was, because the surface has been opened up and given it somewhere better to live.

It shows first on south and east walls, under eaves, behind shrubs, and anywhere a downpipe or an air-conditioning condensate line keeps one patch permanently damp. If a single wall is filthy while the rest of the house is fine, look up rather than across — the cause is nearly always something above it holding water.

None of that means the render is failing. On a sound wall the growth is sitting on top of the coating, not eating into it. Left for years it will hold moisture against the paint and shorten its life, which is the real argument for dealing with it rather than living with it.

What not to do

Six ways people wreck render trying to clean it

A turbo nozzleA rotary or turbo tip will cut render. It leaves crescent-shaped scars, and once the acrylic texture has gone there is no invisible patch — the wall needs recoating.
Full pressure on paintPainted render is a thin film over a porous substrate. High pressure lifts it at the edges, and the damage often does not announce itself until it starts flaking weeks later.
Chlorine straight from the bottleUndiluted pool chlorine bleaches the colour out of tinted render, burns whatever is growing below it, and corrodes aluminium window frames and flyscreens.
Working a hot wall in full sunThe solution dries before it can do anything, leaving streaks and a tide line you then chase with more chemical. Shaded side first, or early in the morning.
Skipping the rinseAnything left on the wall keeps working on the paint long after the job and gives dust something to stick to. The rinse-down is genuinely half the work.
Ignoring the causeClean the wall but leave the blocked gutter, the weeping downpipe or the shrub pressed up against it, and you have simply booked the same job again next year.

The method

Kill it, do not blast it

A soft wash is a low-pressure application of a cleaning solution, a dwell period while it does the work, then a gentle rinse. The pressure sits barely above a garden hose. The chemistry does the cleaning; the water only carries it on and takes it off again.

Dilution is decided by the substrate and by how heavy the growth is — painted render takes a milder mix and a longer dwell than raw brick or Colorbond. Plants are soaked with clean water before and after, windows and screens are rinsed down, and run-off is directed away from garden beds and stormwater rather than left to find its own way.

Done properly, the wall keeps improving for a few days after we leave as the dead growth weathers off. It also stays clean far longer than a pressure-washed wall, because there is nothing left in the render for it to regrow from. House washing starts at $495 inc GST.

Common questions

Mould on render, answered

Will a pressure washer take mould off render?

It will take the top layer off, and often some render with it. The growth is rooted in the surface, so pressure removes the colour without removing the organism and it regrows from what is left behind. It is the single most common way a rendered wall ends up permanently marked.

Can I just use bleach and a broom?

On a small patch at ground level, properly diluted, sometimes. The trouble starts with reach, with run-off into garden beds, and with getting an even result — patchy application on render leaves a striped wall that looks worse than the mould did. Never use it undiluted, and never on a hot wall in direct sun.

Will it damage my plants?

Not if the site is prepared properly. Anything growing near the wall is soaked with clean water before we start and again afterwards, which stops it taking anything up, and beds hard against the house get covered where that is not enough.

How long before it comes back?

On a soft-washed wall in a reasonable position, years rather than months. Shaded southern walls, houses under trees and anything sitting near constant moisture will go sooner. Fixing whatever is keeping the wall wet is what really extends it.

Do the windows and screens get cleaned too?

Everything that gets solution on it gets rinsed down, so you are not left with residue or run marks. A proper pure-water window clean is separate work, and worth booking on the same visit if the glass has not been done in a while.

What if the paint is already failing?

We will tell you before we start. Where render is chalking or the paint has already let go, a wash will take loose material with it no matter how gently it is done. That is a painting job with a clean beforehand, and it is far better to know standing in the driveway than halfway through.

Next step

Not sure if it is mould or a paint problem?

Send a photo of the worst wall. We will tell you which it is and what it actually needs, then quote the job properly. Free, no obligation.

Get your free quote