If you're renovating an older Sydney apartment or house and you've pulled up the carpet or tiles to find a hard, brownish surface underneath, there's a good chance you've found magnesite flooring — and it needs to come out before any new timber flooring goes down. Covering it over might seem like the simpler, cheaper option, but it's one of the most common causes of expensive flooring failure we see in older buildings. Here's why.

What is magnesite flooring?

Magnesite flooring was a common topping applied over concrete slabs in Australian buildings from around the 1920s through to the 1980s, particularly in unit blocks and apartments. It's made from a mix of calcined magnesite and magnesium chloride solution, combined with fillers such as wood flour or sawdust — and in some cases, asbestos was used as a filler material too. It was originally used as a levelling and sound-dampening layer, not intended as a permanent flooring base, which is exactly where the long-term problems begin.

In Australia, it typically appears as a brown or tan coloured layer directly beneath old carpet, tiles or lino, often 40 to 100 years old by the time it's uncovered during a renovation.

Why magnesite causes problems over time

Magnesite is highly absorbent and draws in moisture readily over the decades it's been in place. That moisture doesn't stay contained — it gradually migrates into the concrete slab beneath, and the chloride content in magnesite is corrosive to the steel reinforcement inside the slab. Over time, this corrosion causes the steel to expand, cracking and damaging the surrounding concrete from the inside — a condition commonly known as concrete cancer.

The dangerous part is that this deterioration is often completely invisible from the surface. A magnesite floor can look dry, solid and undamaged while significant structural degradation is happening in the slab underneath it, out of sight until it's already advanced.

Why you can't simply install timber flooring over the top

Laying new flooring — timber, vinyl or otherwise — directly over existing magnesite traps whatever moisture is already present between the two layers, with nowhere for it to go. This accelerates the very chemical breakdown that's already been happening, increasing the rate of corrosion in the slab's reinforcement beneath a floor you've just spent significant money installing.

It also creates a direct problem for the timber flooring itself. Installing engineered or solid timber over a moisture-retentive base like magnesite runs directly against everything a proper subfloor preparation process is designed to prevent — the same cupping, bond failure and board movement caused by any subfloor moisture problem, except sitting on top of a material that's actively holding and generating moisture rather than simply carrying some residual dampness from a recent pour.

Warranty and liability risks

Flooring manufacturers are explicit about this: adhesives, levelling compounds and flooring systems are not warranted when applied over magnesite, or over any residual magnesite left behind after an incomplete removal. If your floor fails because of moisture or movement traced back to magnesite underneath, you won't have a warranty claim to fall back on — the cost of removing the failed flooring, treating the slab, and reinstalling falls entirely on the owner.

In strata buildings, this can become a significant liability question. Many Sydney strata by-laws treat magnesite as part of common property, and if a lot owner installs flooring over existing magnesite that later causes slab damage, responsibility for the resulting repair costs can become a genuine point of dispute between the owner and the Owners Corporation. It's increasingly common for proactive Sydney strata committees to include magnesite assessment and removal in their long-term capital works planning specifically because of this risk.

Asbestos testing comes first

Before any magnesite removal begins, the material must be tested for asbestos. Because magnesite flooring installed before the 1990s may contain asbestos or asbestos-contaminated compounds, disturbing it without proper testing and clearance poses a genuine health risk from airborne fibres. A certified asbestos assessor should test the material and issue clearance before any grinding, cutting or removal takes place — this isn't an optional precaution, it's a legal and safety requirement.

What proper magnesite removal involves

Once asbestos clearance is confirmed, professional magnesite removal typically follows a clear sequence:

1. Removal of the magnesite layer using specialised equipment designed to limit dust and manage the material safely
2. Concrete grinding of the slab beneath, both to prepare the surface and to expose any concrete cancer or reinforcement damage that was hidden underneath
3. Concrete cancer inspection and repair, addressing any corrosion or structural damage found before proceeding
4. Slab preparation and levelling, using a moisture-stable levelling compound suited to the treated concrete, ready for new flooring

Skipping any of these steps — particularly the concrete inspection stage — risks installing beautiful new flooring over a slab that still has an underlying structural problem waiting to resurface.

Why this matters before you choose your timber flooring

If you've found magnesite during a renovation, it needs to be factored into your project planning and budget before flooring selection even begins — not treated as a minor extra step once the floor has already been chosen. The removal and slab remediation process affects your timeline, your budget, and in strata buildings, potentially requires approval processes of its own before work can start.

Getting it right from the start

Magnesite might have been an acceptable solution decades ago, but it was never designed to sit under a permanent timber floor, and the consequences of leaving it in place tend to surface — quite literally — only after significant money has already been spent. Proper removal, asbestos testing, and concrete inspection protect both your new flooring investment and the structural integrity of your slab.

Found magnesite during your renovation, or unsure what's under your existing floor? Talk to us before finalising your flooring plans — we can help you understand what needs to happen before installation and connect you with the right specialists for removal and slab remediation. Visit our Surry Hills showroom or call (02) 9167 3624.

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