I recently had an enquiry regarding a small puddle on a hallway floor, which was presumed to be a penetrating damp issue. On quizzing my client it was also noted that this was mainly observed first thing in the morning, and also sometimes when there was no rain present during the night. I was asked to investigate, find the cause, and also offer some kind of solution.

The property is a 1950’s  semi detached house, with cavity wall insulation, and the floor in question is of solid construction.

The carpet had recently been removed from the front entrance hallway because it had become stained. The floor in question is red in colour and when tapped sounded hollow in places, but consistently as in the sound of the depth, indicative of a floor type screed. Very high readings were noted when the floor was tested with a damp meter, all of this pointed to me that this is possibly a magnesite floor covering.

Unfortunately on the day of the survey, the visible puddle wasn’t evident, but as somebody that has previous experience with this issue first hand I was pretty certain I knew what the issue was, it was just a question of proving it……

The client was contacted, as I needed to confirm asbestos wasn’t present in the flooring, I know of some cases where it is present in magnesite flooring. Tests confirmed there was none present, and a re-visit was booked – so on this visit I hoped to prove the issue, and also inspect the floor to work out repair costs.

First of all I used a humidifier, this is basically the opposite to a dehumidifier, this basically creates lots of humidity and can quickly exaggerate a problem so I can see an issue rather than presume. The area was tented off and the humidifier was switched on, being impatient I kept opening the porch door to see if water was pooling …it wasn’t! I knew it would take a fair amount of time, but in the end it took about 45 minutes for this to occur, unfortunately because of the amount of humidity I didn’t get the video footage showing it occur…I was gutted.


The below picture actually shows the water on the surface, although not a great shot!


This shows the magnesite floor opened up, it was around 25mm in depth in the places inspected.


Now a layperson may wonder why the puddle occurred, and was it a condensation issue?

Condensation was definitely not the cause, this was completely eliminated using our equipment.

Now magnesite floors are basically very hygroscopic. Hygroscopic salts absorb water from the air; a good example is that of sodium chloride, common salt, which under humid conditions crystals become ‘sticky ‘clump together. However, there is an extreme of hygroscopic-deliquescent-where the salts absorb so much water that affectively they dissolve in themselves and thus we have a solution. The point at which deliquescent occurs depends on the particular salt. For example common salt is hygroscopic at 75% relative humidity but increased this to 85% plus and keep it there then it becomes deliquescent and therefore liquid. Once in liquid form the salts become mobile and can move away from their point of origin.

Difference salts become deliquescent at different humidities. Some like calcium chloride and many of the calcium salts deliquesce at very low humidities: others such as sodium chloride become deliquescent at much higher humidities. Whatever the case there are occasionally conditions where the material becomes physically damp solely due to the contamination by such salts.

As there was no floor covering present, and the night  time temperature drops, subsequently the relative humidity increases (high relative humidity in this case  = deliquescent) – this is why the problem was only viewed first thing in the morning.

Tips for surveyors

If you see this type of floor colour it could be magnesite, and this is the only colour I’ve personally ever seen.   I’ve been told that in other locations that there is greens and brown. Also remember that there is also red pigmented sand and cement screeds of this era. This floor in particular was very soft when poked, although some I have found are harder, when in better condition. When checking solid floors through carpet coverings on pre purchase surveys, if you get very high readings, you could put your client on notice, as the repairs can be very expensive.

Also remember to get it tested for asbestos, this is a cheap process and has to be carried out.

Magnesite tends to be quite warm to the to touch, as sometimes it can have a wood filler present.

The two below pictures show macro images taken of two different magnesite floors, as you can see the make up material is completely different.


Magnesite tends to expand under the influence of free lime as an impurity causing cracking. Since it only has moderate strength and is laid bonded and relatively thin it will reflect movements in the base of the surface.

Chlorides may migrate from magnesite screeds into concrete, and accelerate corrosion in structural reinforcement. Magnesite can also corrode all mild steel products including gas and water pipes.

The first magnesite floor was probably laid in Britain around the 1890’s following its introduction from Austria and Germany).

Tips for anyone reading this that has a damp issue.

Any damp survey should be an investigation rather than a quote. You should expect to pay for an investigation also, anybody that offers free surveys is out to sell a product…. In this case prior to us a free ventilation survey was carried out and recommended new extraction fans, and also they had a free cavity wall insulation survey check, and you guessed it the issue was because of damp cavity wall insulation.

If you need survey or advice contact us on 01225 769215.

Posted by Complete Preservation


  1. Some really good advice here, never knew about asbestos being an issue.



    1. Thanks Patrick, always learning….well I am 🙂


  2. Brendan O'Callaghan November 15, 2017 at 21:33

    Fantastic in – depth blog. Thank you


  3. Simon Wilkinson November 17, 2017 at 13:51

    Very informative and useful info as always.


  4. this is all well and good but doesn’t tell anyone how to remove the magnesite flooring, many of which in older properties are over wood sub floors


    1. Thanks Angel for your comments. It’s a case study based on a survey not on the repair unfortunately. I’ve never seen magnesite flooring over a wood sub floor, this isn’t what it was designed for. Are you sure you’ve seen this over many wood sub floors?


      1. Yes, in older houses. I can post some pics if you like (if you can tell me where the button is to add them).

  5. Hi,

    Great information. I am trying to figure out if my floor is magnesite and what to do next if it is.

    It certainly looks like it visually. I got a damp reader which gave me a reading in the ‘middle” range (in the parts of the floor which don’t look damp) – how high do readings tend to be if the floor is magnesite if you don’t mind sharing?

    Thank you very much.


    1. Hi, I can’t really tell you. Normally you would get a very high reading. It could be a pigmented red screed?


      1. Hopefully. Thank you for the reply.

      2. Thank you very much 🙂

        The asbestos test came back negative and considering all the data you are probably right that it is pigmented red screed. Thank you again for the information and the help.

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