Why Does My House Grow Mould Outside — and How Do I Remove It?
Quick answer
Green and black growth on Gold Coast house exteriors is algae and mould feeding on moisture in our humid subtropical climate — it appears fastest on southern walls, shaded sides and textured surfaces like render. The safe removal method is soft washing: low-pressure application of a biodegradable solution that kills the growth at its root, rather than high-pressure blasting, which damages paint and render and leaves roots behind to regrow within months. A professional house softwash on the Gold Coast costs from $495 for a small single-storey home.
Why the south side of your house is always the green one
Algae and mould need two things: moisture and low UV. In South East Queensland, southern and south-eastern walls get the least direct sun, stay damp longest after rain and overnight humidity, and so they green up first — often years before the northern side shows anything. Anything that holds shade makes it worse: dense gardens against walls, boundary fences close to the house, trees, and the textured surfaces of acrylic render, which give spores a foothold smooth paint doesn't.
It's worth being clear that the growth isn't just cosmetic. Mould colonies hold a damp film against paint and render, slowly breaking down the surface, and heavy growth near windows and doors contributes spores to the air coming into the house. Real estate agents will also tell you a green-streaked house is one of the fastest ways to lose buyers at the kerb.
Soft washing vs pressure washing your house walls
House walls should never be high-pressure cleaned. Painted surfaces strip, acrylic render gouges and dents, mortar joints erode, and pressure drives water into weep holes and wall cavities where it causes damp problems you can't see until they're expensive. And because pressure only removes the visible growth — not the root structure — the green comes back within months, often worse because the surface is now scoured and porous.
Soft washing works the opposite way: a biodegradable cleaning solution is applied at low pressure (comparable to a garden hose), dwells long enough to kill mould, algae and lichen at the root, and is rinsed gently. The result typically lasts 2–4 times longer than pressure cleaning, and the paint, render and seals underneath are untouched. It's the method paint manufacturers recommend before repainting, too — many of our house washes are pre-sale or pre-repaint preparation.
What a house softwash costs on the Gold Coast
Guide prices: from $495 for a small single-storey home or small office, $770 for a large single-storey or small double-storey, $990 for an XL single or large double-storey, and $1,200+ for XL double-storey homes and beyond. That covers walls, eaves, and exterior surfaces — and we pre-wet and rinse gardens as standard to protect your plants.
How often? Most Gold Coast homes look freshly painted again with a softwash every 2–3 years. Shaded, south-facing or heavily gardened homes lean toward every 1–2 years. Doing it before growth is established is noticeably cheaper than letting lichen colonise render — lichen takes more product, more dwell time and more care.
Can I remove exterior mould myself?
Small reachable patches — sure: diluted household bleach or a commercial exterior mould killer, a soft brush, and a thorough rinse will handle a metre of green near the front door. A whole wall or a whole house is a different job: you can't get even coverage from the ground, ladders plus wet chemicals are a bad combination, garden protection matters at scale, and the DIY result tends to be clean patches with green halos. If a wall has gone properly green, it's usually softwash territory.
Related Questions
Usually both — black mould colonies bonded into a film of grime. Quick test: press a gloved wet thumb on it. If it smears slightly or the surface feels slimy after rain, it's biological. Either way soft washing removes it; what matters is that it's treated chemically, not blasted.
No — protecting gardens is a standard part of the process. We pre-wet garden beds before applying solution and rinse everything thoroughly afterwards. The solutions we use are biodegradable and break down quickly. If you're on tank water, tell us when booking so we can disconnect downpipes first.
A typical single-storey home takes 2–3 hours; a large double-storey 3–5 hours. Walls are safe to touch once dry, and the growth keeps breaking down for days after — soft-washed surfaces often look even better a week later.
Before painting: essential — paint over mould and it grows through the new coat, and most paint warranties require a clean substrate. Before selling: it's one of the highest-return presentation jobs going; a few hundred dollars makes the whole house read as 'well maintained' in listing photos and from the kerb.