Mould on external walls in Brisbane: causes and how to treat it
Exterior mould and algae grow where walls stay damp and shaded, which is common in Brisbane's humid climate. They're treated by killing the growth at the root with a suitable solution and a low-pressure rinse, then reducing the dampness that feeds it: clear gutters, trim plants back and improve airflow.
Black, green or grey patches on the outside of a house are almost always mould, algae or both. They're mostly a surface and appearance problem, but heavy growth can hold moisture against paint and render and make them age faster.
Why it grows
- Humidity and warmth. Brisbane's climate suits growth for much of the year.
- Shade and poor airflow. South-facing walls, narrow side passages and walls behind dense plants stay damp.
- Water coming from above. Overflowing gutters and leaking downpipes keep walls wet.
- Porous surfaces. Render, unsealed brick and old paint hold moisture.
How to treat it properly
- Protect plants and outlets.
- Apply a cleaning solution suited to the surface so it kills growth at the root.
- Give it time to work.
- Rinse at low pressure. High pressure can strip paint and push moisture into the wall.
Cleaning only the surface with a blast of water removes the colour for a while, but leaves the growth that regrows quickly. That's why treating at the root matters.
Stopping it coming back
- Keep gutters and downpipes clear. See our gutter guide.
- Trim plants away from walls so air can move.
- Fix leaking taps and downpipes.
- Re-clean before growth becomes heavy.
When to get advice
Exterior growth is mainly cosmetic. If mould is inside the home, or someone in the household has asthma or allergies, get advice from a health professional or a building inspector. Persistent dampness can point to a leak or drainage problem that cleaning alone won't fix.
Questions, answered
Is black mould on my walls dangerous?
Outdoor mould and algae are mostly a surface and appearance issue, but heavy growth holds moisture against the wall. If you have health concerns, or the mould is inside the home, seek professional advice.
Will bleach remove it?
It can lighten the growth, but it may not kill it at the root and can harm plants and some surfaces. A suitable treatment applied at low pressure is a safer approach.
How long before mould comes back?
It depends on shade, trees and airflow. Shaded, damp walls return sooner than sunny, dry ones.
Need mould treatment?
Get a clear, itemised quote, usually within one business day.
