How to remove mould from your house exterior

If your walls have gone green, grey or blotchy black over winter, you are not alone. Canberra's cold, damp winters and long stretches of shade are ideal conditions for mould, mildew and algae to take hold — especially on south-facing walls that never quite dry out. It shows up on rendered walls, painted brick, weatherboard, fibro and cladding, and even along the ribs of Colorbond fencing and sheds. The good news is that getting it off is straightforward once you know the golden rule: this is a gentle job, not a blasting job.

Why exterior mould is worth dealing with

Beyond making an otherwise tidy home look tired and neglected, a film of mould and algae can slowly degrade paint and render if it is left to sit season after season. It holds moisture against the surface, and where it creeps onto paths and steps it becomes genuinely slippery. There is also an air-quality angle: spores near windows, doorways and entries can drift indoors, which matters if anyone in the house is sensitive to mould. Clearing it keeps the place looking cared for and the surfaces lasting longer.

The one rule that matters most

Do not point a high-pressure lance at render, painted surfaces or cladding. High pressure etches soft render, strips paint off timber and fibro, splinters weatherboard and — worst of all — can drive water in behind the cladding, where it sits and causes far bigger problems than a bit of green ever would. Mould removal on a house exterior is a soft-washing job. If you want the full comparison, we cover it in our guide to soft washing vs pressure washing.

The correct soft-wash method

Soft washing works by chemistry rather than force. The sequence is simple:

1. Apply a mould-killing cleaning solution to the affected area, working from the bottom up so you can see where you have covered.
2. Let it dwell for around 10 to 15 minutes. This is the step most people skip, and it is the one that counts — the solution needs time to kill the growth right at the root rather than just lifting the surface layer.
3. Agitate gently with a soft brush if there are stubborn patches. No scrubbing hard enough to mark the surface.
4. Rinse at low pressure with a normal hose or a wide, gentle spray.

The reason this outlasts a quick blast is simple: killing the mould means it stays clean for much longer, where a pressure wash only removes what you can see and leaves the roots behind to grow straight back.

What to clean it with

Dedicated exterior mould and house-wash treatments — often sodium hypochlorite based — are the common choice because they kill mould effectively. If you would prefer something milder, oxygen bleach or a suitably diluted household solution will handle lighter growth. A few things to keep in mind:

• Bleach-based solutions can harm plants and lawn. Wet surrounding plants down before you start and rinse them again afterwards, and avoid letting overspray drift onto the garden.
• For a small, mild patch you do not need a pressure washer at all — a soft brush, a suitable cleaner and a garden hose will do the job.
• Always follow the product's dilution directions; stronger is not better and can damage the very surface you are trying to save.

Stopping it coming back

Mould returns wherever walls stay damp and shaded, so a bit of prevention buys you a lot of time between washes:

• Trim back vegetation near walls to improve airflow and let more sun reach the surface.
• Fix leaks and downpipes that keep a section of wall permanently wet.
• Keep gutters clear so overflow is not constantly running down the render or weatherboard.

Staying safe

Most of the risk in this job is not the mould — it is the ladder. Working at height is where people get hurt, so keep both feet planted and do not overreach. Wear eye protection when you are applying cleaning solutions, and keep plants and pets clear of the chemicals until everything has been rinsed down.

When to call a professional

Some jobs are better handed over. Two-storey walls, large areas, or delicate and heritage render where you cannot risk any damage are all good reasons to bring in a soft-wash pro who has the reach, the right dilution and the experience to get it right the first time. If that sounds like your place, grab an instant estimate and we will take it from there.

FAQs

What kills mould on render?

A mould-killing house-wash treatment applied and left to dwell, then rinsed — not high pressure. Letting the solution sit for around 10 to 15 minutes lets it kill the growth at the root so the wall stays clean longer.

Can you pressure wash render or painted walls?

No — high pressure damages them. It can etch render, strip paint and force water behind cladding. Use a soft wash instead: a cleaning solution that dwells and kills the mould, followed by a low-pressure rinse.

How long until the mould comes back?

It depends on how much shade and damp the wall gets. Improving airflow, drainage and keeping gutters clear all slow regrowth. A treatment that actually kills the mould lasts longer than simply blasting the surface layer off.

Does bleach kill exterior mould?

Yes, chlorine-based solutions kill it, but protect plants and rinse them afterwards. Milder oxygen bleach is an option if you would rather avoid chlorine near gardens.