Magnesite floors

On April 16, 2016



Magnesite (magnesium oxychloride) flooring was used widely after the second world war as a concrete topping and self-finish floor screed.  The material is both hygroscopic and corrosive.  A migration of salts from the magnesite is known to cause accelerated corrosion of steel reinforcement within the floor slab.  This can also be a problem in steel framed buildings with stancheons.  Persistent wetting and very humid internal environments can cause the material to hold onto moisture.  This moisture can cause the magnesite to break up and deteriorate, as seen here.


This can only be confirmed by laboratory analysis.  It is common for people to confuse regular pigmented sand and cement screeds with magnesium oxychloride ones.  There are however some simple ways to help identify magnesite floors:

  •  Magnesite floors are usually either light pink or yellow in colour.
  •  An electrical resistance moisture meter will always give a high reading.
  •  The material will give off a bright glow when exposed to a naked flame.


The material will deteriorate when wet.  It’s important therefore to assess the integrity of any damp proof membrane within the concrete slab.  This is something that could be checked using a floor hygrometer.  Magnesite floors are usually 15-25mm thick.  In contrast, regular sand and cement screeds are usually 50-75mm thick.  Therefore, hacking up and re-screeding the floor will usually result in an increased height in finished floor level.  This will mean that there will be some alterations to the internal joinery (ie. timber skirtings, doors and door frames, etc).  To get round this problem, an accepted method would be to use a non-traditional floor screed that can be laid in much thinner coats.  This could be asphalt to BS9825:1988 for example.  Another solution could be to use a special polymer modified cementitious screed.  The asphalt method would eliminate the need to install a new DPM, should it be found that one is required.  It should be noted that there would be a risk that some low-level dampness would appear in the perimeter walls, after the new screed is laid.  This dampness, if it occurs, would be due to the displacement of moisture that comes up through the slab from the ground.

In some cases, asbestos could have been added to the magnesite.  A bulk sample should therefore be sent away to a laboratory, so that this can be confirmed.  This testing should be done before the existing screed is disturbed.  If asbestos is found, then the magnesite would need to be removed by a licensed removal contractor. The replacement of a floor screed could be expected to cost something in the region of £65-80 per square metre.  An additional provisional sum should be included for any enabling works and general making good of the floor slab.

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