Magnesite Floor Failure

Property In London W7, with Magnesite Floor Failure
Property In London W7, with Magnesite Floor Failure

I’ve recently just completed a long Roadshow for the RICS, ironically, I was giving a talk on solid floor defects, and included a section on Magnesite floor failure, as a refresher. In the last talk I gave, I asked an audience of about 70 Chartered surveyors, how many had seen a magnesite floor recently; the answer was none. I myself have not seen for for quite a few years but only a week later, I attended a property in London to investigate an alleged problem with sulphate attack, and guess what I found?…

What is Magnesite Flooring?

Magnesite, or Magnesium Oxychloride, was a product widely used by the flooring industry between circa 1920 and 1960. It was especially used in industrial premises, as it was resistant to oil spillages; however, it was frequently used in residential premises.

It is a water based product, commonly reddish pink in colour, though can be pale yellow, or any other colour specified.

Magnesite Flooring
Magnesite Flooring

Magnesite flooring was made from a mixture of calcined magnesite and magnesium chloride solution with various fillers (e.g. wood flour, sawdust, asbestos).• It was typically laid between 10 and 25mm thick, but two coat applications could be up to 50mm thick. 

What’s wrong with Magnesite?

Firstly, Magnesite contains chlorides, so if there is any embedded steel reinforcement within the floor slab, then the concrete can be affected by Chloride attack, which will corrode the embedded steel. Corrosion is an expansive reaction, and cracking of the concrete is likely to occur, as the steel corrodes. You should also consider, that steel water pipes may be buried in the concrete, and these are equally at risk.

Many old concrete floors, do not have a damp proof membrane installed, DPM’s came into common use in the mid 60’s, but prior to this, many concrete floors had a waterproof oversite, a layer of bitumen was commonly used. It would be wrong to assume that Magnesite provides that same protection against damp, and in fact they are very vulnerable to dampness. Magnesite is water soluble, and will return to its previous state if exposed to enough water.

The Asbestos Risk

As discussed earlier, Magnesite can contain asbestos fibres, as a filler. Commonly, the way to deal with asbestos, once identified, is to remove it, using a licensed contractor, or to encapsulate it. However, you can’t encapsulate a Magnesite floor, as they are so vulnerable to deterioration when exposed to water. Obviously, if you tried to encapsulate by pouring a screed over the top, then you’d be introducing large amounts of construction moisture into the Magnesite. The underlying Magnesite, would then most likely turn to a Weetabix type consistency, and start to break up, leaving you with no suitable substrate support below the screed.

Magnesite Floor Case Study

In this particular property, a Chartered surveyor had recently attended, for a pre-purchase survey and noted heave, or an uneven concrete floor below the carpets; he then of course raised the alarm for a potential risk of sulphate attack.

I attended to sample the floor, but on pulling up the carpet, the cause of this uneven floor, was clearly Magnesite floor failure; the Magnesite having got saturated, subsequently expanding and causing large blisters in the floor.

It was still important to investigate the situation with the underlying concrete and I excavated a hole through the slab to sample both the concrete, and the underlying hardcore. However, on breaking through the 8″ thick concrete slab, we found that there was no hardcore, and the slab sat directly on wet clay, with no DPM installed. This of course means that the concrete is in direct contact with ground sulphates.

Concrete slab sitting directly on clay
Concrete slab sitting directly on clay

The concrete was also notably wet, and this moisture had transferred to the Magnesite, causing it to heave up, blister, and crumble. From the image below, you can see how the magnesite had delaminated from the concrete substrate, forming large blisters, which crumbled when you stepped on them.

Failed Magnesite Flooring
Failed Magnesite Flooring

We did take samples of the concrete for sulphate tests, but with a saturated slab and widespread failure of the Magnesite, my advice was to renew all the solid floors, with the only test required being for asbestos. Testing the magnesite for asbestos, was critical before any works to remove the flooring could proceed.

One final note worth mentioning for any surveyors looking to check for dampness in Magnesite. Magnesite is electrically conductive, so if using a hand held electronic moisture meter, it will always give a high reading for damp.

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  • malcolm Holland

    There are quite a few of these in my “patch” in Bletchley from around 1930s. This pretty much describes them spot on in my limited experience. Common to find blisters as sawdust filler expands with moisture

    • Joe Malone

      Interesting comments malcolm; I hadn’t really considered that the sawdust filler might be a significant co-factor in the expansion. The sample is currently away for asbestos testing, though to the naked eye, it looks like a wood filler; so I expect that you’re exactly right on this.

  • Kirsty W

    Interesting and informative, as always! Not seen any magnesite floors in my area in Cornwall, but I’ll keep an eye out.
    This is such a useful blog for graduate surveyors like myself, thank you.

    • Joe Malone

      Thanks very much for the kind words Kirsty, it’s good to hear that graduate surveyors are finding this blog useful.

      Regards. Joe

  • Gmac

    Great insite . Unfortunately we also have came across this in an old building sample came back as positive and will now be removed by a specialist.

  • John McNamee

    I have just discovered this type of flooring in my kitchen after taking up the laminate. I also have a blister as you described beneath the sink unit. The floor is extremely uneven so how do I get it up or should I overlay it?

  • Chris Miles-Witchell

    As a flooring contractor of nearly 40 years, i have seen quite a few of these.They are most often, (as pic), red, but can come in many colours, also seen it in black.Can only be left, if covering with a breathable overlayer, such as carpet.Anything else, such as vinyl & it will often begin to `blow`.

  • Ali R

    Hi, just curious as to whether the test for asbestos came back positive?
    We’ve discovered this in our 1880sish kitchen under some lumpy parquet flooring (the kind laid in prestuck tiles over a layer of mdf) and it seems to have a significant amount of sawdust between it and some concrete. We’ll be sending it off for tests at some point, everything’s stalled at the moment due to covid.

    I’m not sure when the concrete was laid but it has some large cracks in so it was possibly an early addition with the magnesite.. I want to excavate more to find out if the concrete is also failing but very worried about the costs of relaying and whether digging up the slab will affect the house itself! It’s very solid but old!
    So also wondering if you had to renew the solid floors in this case!

    • Joe Malone


      The asbestos test on this sample came back as negative, but you shouldn’t read anything into that, with regards to your own situation.
      I had no further involvement so don’t know if the floors were renewed, though I’d imagine they were.

      Regards. Joe Malone

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