Why Building Mould Spreads — and How to Stop It Coming Back

Mould on the exterior of a building is one of the most common complaints strata managers and property managers receive — and one of the most misunderstood. Many properties get cleaned, look great for a few months, then develop the same black and green streaking again. Understanding why this happens is the key to stopping it.

Why Mould Grows on Building Exteriors

Exterior mould, algae, and lichen are biological organisms that need three things to grow: moisture, a surface to attach to, and organic material to feed on. Most building exteriors in NSW provide all three.

Rendered walls, painted surfaces, concrete, brick, timber, and roof tiles all retain some moisture and collect fine particles of dust, pollen, and organic debris over time. In shaded areas that don't dry out fully after rain, mould spores find ideal conditions and colonise quickly. Once established, a mould colony spreads outward — it doesn't stay where it starts.

In coastal and high-humidity areas of NSW, growth accelerates significantly. Properties surrounded by trees, with north-facing walls in shade for much of the day, or with poor drainage around the building perimeter, are particularly prone to rapid re-growth after cleaning.

Why It Comes Back After Cleaning

Pressure cleaning alone removes the visible surface growth but doesn't always eliminate the root structure of the organism. Algae and lichen in particular attach to surfaces at a microscopic level, and high-pressure water alone may not reach these attachment points.

The more effective approach for most exterior surfaces is soft washing — using low pressure combined with a professional-grade biocidal treatment that kills the organism at the root and leaves a residual protective effect on the surface. A properly treated surface resists re-growth for significantly longer than a pressure-only clean.

The other common reason mould returns quickly is that the underlying condition — shade, moisture, poor drainage — hasn't been addressed. Cleaning treats the symptom; the environment that caused it remains unchanged.

Surfaces That Are Most Vulnerable

• Rendered and painted walls — especially in shaded aspects
• Roof tiles — both terracotta and concrete, particularly in valleys and northern aspects
• Timber cladding and weatherboard — porous surfaces retain moisture
• Eaves and soffits — trap moisture and receive little direct sunlight
• Concrete pathways in shaded areas — green algae builds quickly and becomes a slip hazard
• Fence panels and boundary walls adjacent to garden beds

How to Actually Stop It Coming Back

The combination that works:

• Soft washing with biocidal treatment rather than pressure-only cleaning on affected surfaces
• A scheduled maintenance cycle that treats surfaces before growth becomes heavy — lighter treatments more frequently outperform heavy restoration cleans every few years
• Addressing drainage issues or excessive shade around the building perimeter where practical
• Ensuring gutters and downpipes are clear so water isn't sheeting down exterior walls after rain

For strata and commercial properties, a biannual soft wash programme for the building facade, combined with quarterly attention to high-risk areas like bin zones and shaded pathways, is the most effective way to stay ahead of mould rather than constantly reacting to it.

What to Expect From a Professional Treatment

A professional soft wash for mould should include a pre-treatment inspection, application of an appropriate biocidal solution suited to the surface, low-pressure rinse, and a post-clean assessment. On heavily affected surfaces, some residual staining may remain after the first treatment as the dead organism breaks down — this typically clears within a few weeks of rainfall.

For heavily colonised surfaces, a second treatment six to eight weeks after the first clean often achieves noticeably better long-term results than a single heavy clean.