Fibrous cement sheeting, universally known in Australia as “fibro”, was the default cladding and internal lining material for residential construction across New South Wales from the 1930s through to the 1980s. In Northern Rivers towns like Lismore, Casino, Murwillumbah, and Kyogle, fibro homes make up a substantial proportion of the older housing stock. These homes present a distinct set of challenges when it comes to mould: the building system itself creates conditions that are particularly hospitable to mould growth in the Northern Rivers subtropical climate.
Quick answer (BLUF)
Fibro homes in Northern Rivers develop mould both on external and internal surfaces because of the many gaps, joins, and moisture pathways in older fibrous cement construction. The challenge is compounded by the potential presence of asbestos in pre-1987 fibro, which means any investigation or treatment that involves disturbing fibro sheeting must comply with asbestos safety regulations.
Why fibro homes struggle with mould
Fibrous cement sheeting itself is not particularly porous, the material is relatively dense and water-resistant. But a fibro home is not just its sheeting: it’s a system of sheets, battens, joins, sealants, and framing, and it’s that system that creates mould vulnerability.
Joins and gaps: Fibro sheets are joined at edges, windows, doors, and wall-ceiling junctions. Over decades, sealants fail, joins open, and these gaps allow moisture to track into wall cavities. In a Northern Rivers wet season, driving rain against external walls finds every gap, and internal condensation in the subtropical climate means moisture is also moving from inside the wall outward. Where the two moisture sources meet, in the wall cavity, mould conditions are created.
Timber framing: The structural framing behind fibro sheets is timber, typically hardwood or softwood depending on the era of construction. This framing is the primary mould substrate. In an older fibro home with inadequate vapour management, the framing can remain damp for extended periods, sustaining mould growth that is entirely invisible from either the interior or exterior surface.
Lack of vapour barriers: Pre-1980s construction in Australia generally did not incorporate vapour barriers or sarking in wall systems. Without a moisture barrier, wall cavities have no protection against moisture movement from either direction.
Ground contact: Many older fibro homes in Northern Rivers were built on low stumps or on ground-bearing footings with limited subfloor clearance. Direct or near-direct soil contact creates moisture pathways into the wall base, and mould at the base of fibro walls, both internally and externally, is very common in these properties.
The asbestos complication
This is the defining complication for fibro homes built before 1987. Approximately 60 percent of fibro sheeting installed in Australian homes before 1987 contained asbestos, chrysotile (white asbestos) in the majority of cases, but occasionally crocidolite (blue) or amosite (brown) in some products. The Northern Rivers has a significant stock of pre-1987 fibro homes.
The critical implication for mould treatment is this: any mould investigation or treatment that involves disturbing, drilling into, cutting, or removing fibro sheeting in a pre-1987 home must be conducted with asbestos safety protocols in place.
This affects mould assessment as well as treatment. A moisture meter reading through undisturbed fibro sheeting is safe; drilling through it to access a wall cavity is not (without asbestos clearance). Any remediation that involves removing or cutting fibro sheets requires the involvement of a licensed asbestos assessor and potentially a licensed asbestos removalist.
For mould assessment purposes, air sampling and careful visual inspection of accessible areas is the starting point. Many Northern Rivers mould inspectors working on fibro properties have experience working within these constraints.
Surface mould on fibro: treatment approach
Mould growing on the surface of fibro sheeting, on external walls, in bathrooms, on ceiling sheets, is the most common and most treatable form. Unlike some substrates, fibrous cement does not absorb surface moisture in the same way timber does, so surface mould on fibro is often genuinely limited to the surface film rather than penetrating into the material.
Treatment of surface mould on fibro involves:
- Identifying and sealing any moisture source driving the surface dampness
- Careful removal of loose mould from the surface using appropriate antifungal cleaners
- Application of a mould-inhibiting primer before repainting
If the fibro sheeting has been repeatedly saturated (for example, from persistent roof leak or rising damp), the cement matrix itself can soften and the integrity of the sheet may be compromised. Soft, crumbling, or flaking fibro that has mould should be assessed by a professional before any treatment or disturbance.
When mould is inside the wall cavity
For fibro homes where mould is suspected inside wall cavities, indicated by musty odour without visible surface growth, persistent mould recurrence despite surface treatment, or known moisture entry points, investigation and remediation becomes significantly more complex.
In a non-asbestos fibro home, carefully cutting an inspection port in the sheeting to access the cavity and assess the extent of mould in the framing is possible. In a pre-1987 home, this must be preceded by asbestos testing and managed under safe work practices.
Professional mould inspection using thermal imaging and moisture meters can often identify wet zones and probable cavity mould locations without requiring penetration of the sheeting, a useful first step that avoids unnecessary disturbance.
FAQs
Do I need to test for asbestos before treating mould in my fibro home?
If your home was built or clad with fibro before 1987, you should assume the sheeting contains asbestos until tested otherwise. Surface mould treatment that doesn’t disturb the sheeting, cleaning and repainting, is generally low risk, but any investigation or remediation that involves cutting, drilling, or removing fibro requires asbestos clearance. Always confirm the situation with a licensed asbestos assessor before proceeding.
Can I paint over mould on external fibro to stop it coming back?
Painting over mould without proper treatment will not stop it returning, the mould beneath will continue growing and the new paint will peel or discolour within months. The correct sequence is: treat the moisture source, clean the surface mould, prime with a mould-inhibiting primer, then repaint. Skipping any step leads to rapid recurrence in Northern Rivers’ climate.
My fibro home in Lismore flooded in 2022. What do I need to know about mould and asbestos?
Flood-affected fibro homes present a particularly complex dual hazard. Floodwater can cause fibro to soften and release asbestos fibres, particularly in older, deteriorated sheeting. If your home is a pre-1987 fibro and was flood-affected, a combined asbestos/mould assessment is strongly recommended before any cleaning, remediation, or renovation work. This is not optional, it’s a regulatory requirement and a health necessity.
Is mould in a fibro home harder to fix than in a brick or timber home?
Yes, in some respects, particularly where asbestos is present, which limits investigative access and increases the compliance cost of remediation. However, surface mould on fibro is in some ways more straightforward than mould in porous materials like plasterboard or raw timber, because the sheeting itself doesn’t absorb mould deeply. The real challenge is usually what’s behind the fibro, not the sheeting surface itself.