Magnesite flooring hides under carpet in older Sydney apartments and can trigger concrete cancer when it gets wet. What it is, how to spot it, who pays in strata, and 2026 removal costs.
If your building was built between the 1920s and the early 1980s, there is a fair chance it has magnesite flooring hidden somewhere under the carpet — and most owners have no idea it is there until it causes a problem. Magnesite is one of the classic hidden defects in older Sydney strata buildings: harmless while it stays dry, but capable of quietly destroying the concrete slab beneath it once water gets in.
This guide is for Sydney strata managers, owners corporation committees and building owners who have found magnesite — or been told they have it — and need to understand what it is, whether it is causing concrete cancer, who pays for the fix, and what removal and remediation cost in 2026. It is written by a Class 2 DBP-registered remedial builder, because the removal is only half the job: the concrete underneath usually needs repair too.
Magnesite (magnesium oxychloride) is a cement-like topping made from magnesium oxide mixed with a magnesium chloride solution. From the 1920s to the 1980s it was poured over concrete slabs as a cheap, lightweight levelling layer and acoustic underlay — smooth, seamless, and easy to lay. It usually sits unseen beneath carpet, vinyl, tiles or timber, so a floor that looks completely normal can have a layer of ageing magnesite underneath it.
You often cannot tell until the floor covering is lifted. The tell-tale signs are a hard, cementitious grey or reddish-brown topping under the carpet, sometimes with a slightly fibrous texture; rust staining bleeding up through the floor or on the ceiling below; a persistent damp or musty smell; and lifting or corroding metal door frames and skirtings where they meet the floor. Buildings from the pre-1980s era with concrete slabs are the prime candidates. If your building is that age and has never had its subfloors checked, it is worth a look before a leak forces the issue.
Yes — when it gets wet. Dry magnesite is stable. But the magnesium chloride in it is hygroscopic (it draws in moisture), and once water reaches it — from a plumbing leak, a balcony or bathroom leak, or rising damp — it releases chlorides that migrate into the concrete slab. Chlorides are one of the most aggressive triggers of reinforcement corrosion: the steel rusts, expands, and cracks the concrete from the inside. That is concrete cancer, and left alone it spreads through the slab and into the building's structure. A magnesite floor that has been wet is therefore not just a flooring problem — it is a structural one.
In most NSW strata buildings the concrete subfloor and original magnesite topping are common property, so removing the magnesite and repairing the slab is generally the owners corporation's responsibility — not the individual lot owner's. There is an important caveat: some schemes have passed by-laws that shift responsibility for floor finishes (and sometimes the magnesite itself) onto lot owners, and this is becoming more common. Insurance rarely helps either — most strata and contents policies treat magnesite as a pre-existing defect or gradual deterioration and exclude the removal, though damage to the concrete slab may sometimes be covered. Always confirm the position against your registered strata plan, by-laws and policy before deciding who funds the work. This is general information, not legal advice.
Costs depend on the area, the floor covering above, how much concrete repair is needed, and access. As a 2026 guide, magnesite removal and disposal alone typically runs a few thousand dollars for a single unit, while a unit that also needs concrete cancer repair to the slab can reach $10,000–$40,000. Whole-building programmes across many units run into six or seven figures. Every floor is scoped after inspection.
| Scope | What it covers | Indicative 2026 cost |
| Removal + disposal only | Dust-controlled grinding out of the magnesite, waste removal and disposal, slab left ready for new floor — only where the slab is sound | $3,000–$8,000 per unit |
| Removal + concrete cancer repair | Above, plus breaking out spalled concrete, treating corroded steel and rebuilding with repair mortars | $10,000–$40,000+ per unit |
| Whole-building programme | Multiple units staged, shared access and containment, staged verification | $150,000–$1M+ |
These are indicative ranges to help committees budget, not a quote. The biggest cost drivers are how far the concrete has corroded, the floor covering that has to come up and go back, and dust containment in occupied units.
A durable job removes the magnesite, then repairs and protects the slab it was hiding. The sequence we follow is: lift the floor covering and confirm the extent of magnesite and any slab damage; grind out the magnesite under dust-controlled, contained conditions and dispose of the waste; break out any spalled or chloride-contaminated concrete, clean and passivate the exposed steel, and rebuild with polymer-modified repair mortars to AS 3600; where moisture was the trigger, fix the source (the leak, the failed waterproofing) so it cannot recur; then reinstate the floor. Skipping the concrete repair — just grinding the floor flat and re-covering it — leaves the corrosion active under a brand-new floor.
Most companies that advertise magnesite removal are flooring or grinding specialists: they take the magnesite out and leave a clean slab, which is fine if the concrete underneath is genuinely sound. The problem is that by the time magnesite is being removed, the slab often is not sound — the same moisture that made the magnesite a problem has usually started the concrete cancer. A single-trade removalist is not registered or equipped to repair the slab, so the job stops at the grinding and the structural problem is left for someone else to find later. A remedial builder handles both under one contract, which is why the removal and the concrete cancer repairs in Sydney should be scoped together.
Start with an inspection, not a grinder. Get the floor and the slab beneath it assessed so you know whether it is a straightforward removal or a concrete-repair job — then scope it once, properly.
Atomic Projects is a Class 2 DBP-registered remedial builder that has delivered more than $20M in remedial work across 100+ Sydney strata buildings. If your building has magnesite flooring, our team handles the whole job — dust-controlled removal, concrete cancer repair to AS 3600, fixing the moisture source, and reinstatement — under a single DBP-registered contract. Get in touch for a no-obligation assessment.
Send photos, the engineer's report, or just the symptoms — whatever you've got. A registered builder reads it and calls you back. No call centre, no obligation.