The Problem with Calcium Sulphate Floor Screeds – Damp Floor



New development with floor screed failures

Malone Associates Ltd, were recently instructed by a large National developer to help resolve a dispute relating to one of their London developments. Essentially we were asked to investigate issues relating to ongoing dampness within a newly laid floor slab within a flat in a new low rise block. The history surrounding this matter was that a client had purchased the flat then noticed air blisters under the Amtico flooring in her lounge. The Amtico was taken up and a square section of the floor screed was excavated to see what was going on. My understanding was that moisture was found sat on top of the plastic resilience layer, the floor was patch repaired and further drying was being carried out and monitored until such time as readings were obtained that were low enough to justify laying the finished flooring.

We were unsure what methods of testing and monitoring was being carried out but had early concerns that the wrong type of equipment was being used.

On initial enquiries as to the type of floor screed used we were informed that Calcium Sulphate floor screed has been laid, specifically a Supaflo pumped screed by Cemex.
 It is worth noting at this point that the terms applied to Calcium Sulphate floor screeds can get confusing because they can also be commonly referred to as ‘Calcium Sulphate screed’, ‘Gypsum’ screed or ‘Anhydrite’ screed. All three terms refer to the same generic product.

Site Observations

We attended site in November and after initial visual inspection assessed the flat for condensation risk. To that end the following readings were recorded. Relative humidity = 47%, Ambient air temperature 19.2OC, Dew point temperature = 7.2OC. The floor temperature was recorded at 17OC or roughly 10OC above dew point temperature. These results proved conclusively that there was no active condensation damp problem in the flat on the day of inspection.

It could be seen on initial visual inspection of the floor screed was stripped of finished Amtico flooring and attempts had been made to strip the screed of flooring Adhesive and primer. This is a difficult process and it was clear that there were still patchy areas of adhesive residue across the lounge floor. We also noted a large patch repair area to the floor and noted the dissimilarity between the existing screed and the repair material. It was initially assumed by site staff that the repair section was calcium sulphate and therefore compatible with the rest of the floor. When I asked for this to be confirmed the staff member who repaired the floor confirmed that it was in fact concrete and as such this repair section is incompatible with the existing floor screed and would need to be excavated and replaced. This is because the Tricalcium Aluminate present in ordinary Portland cement (OPC), contained within the concrete repair section, will react with the sulphate in the floor screed and cause ettringite to form. I’m sure you will all recognise this problem as Sulphate attack, which is an expansive reaction and could well lead to problems with the finished floor in the near future.

Test Methods

We noted the presence of a Tramex CME (Concrete Moisture Encounter) within the flat and it became apparent that floor moisture readings were being obtained by this means. This is standard industry practice because these meters are specifically calibrated for concrete and concrete floor screeds. Indeed, we use a Tramex CME ourselves but we are aware that the tool has some limitations when it comes to a reliable diagnostic process. We took electronic moisture readings across the whole of the lounge floor to obtain comparative readings and our own readings tended to confirm and support the previous statement made by the developer, to the effect that moisture levels were highest nearer the patio doors leading to the balcony. We received our highest Tramex reading of 7.5% at this point, which would tend to indicate that the floor was significantly damp. We also noted a very high Tramex reading of 6% to the concrete repair patch. We were looking to obtain readings of 0.5% total moisture content (TMC) or <=75% equilibrium relative humidity readings (ERH). Some of the more attentive readers may have already spotted the potential problem with the use of the Tramex on a Calcium Sulphate floor screed… Did you note that I said they were calibrated specifically for concrete? Calcium sulphate is not concrete and therefore any readings obtained with the Tramex can only be referred to as a relative reading. Tramex’s own product information states that ‘The Concrete Encounter utilises “state of the art” electronic technology to provide the flooring industry with an accurate and simple to use non-invasive handheld instrument for nondestructive testing (NDT) of Moisture Content (MC) in concrete and comparative moisture readings in, gypsum and other floor screeds.’

Moreover, the Tramex is only reasonably reliable when used on virgin concrete that is clean and dust free. Once primers, adhesives and other coatings have been applied to the floor then the Tramex becomes even less reliable. The floor in question had too much surface contamination for the Tramex to ever be reliable even it was concrete as opposed to Calcium Sulphate flooring.


Because the floor repair section was to be excavated and repaired we decided to test the floor using the calcium carbide meter. For those of you not familiar with calcium carbide testing, a small amount of test material is weighed and placed into the pressure vessel. This is then mixed with calcium carbide powder and the chemical reaction within the vessel releases acetylene gas that pressurizes the vessel and causes a reading to register on the gauge. The amount of acetylene produced is directly proportional to the amount of moisture present in the sample and this is the most reliable site test in use for establishing the moisture content of concrete and masonry products. The concrete repair section was indeed still damp, though this was a null point due to its excavation. We simply tested this area to complete our understanding of what moisture contents were actually present right across the floor compared to readings obtained with the Tramex CME. In fact the calcium sulphate screed proved to be bone dry with readings no higher than 0% TMC obtained.

Floor screed testing

Floor screed testing: Relative readings indicate that floor is wet when in fact it is dry.

We believe that the Amtico flooring was initially laid too early but subsequently after removal of the Amtico the developer has dried the floor thoroughly but been chasing their own tail due to false positive readings obtained from the Tramex.

After the floor slab was repaired we provided the client with ongoing advice for monitoring the deep repair section that was replaced but there are only three reliable ways of obtaining reliable results for moisture content in floor slabs, these are:

1. Calcium carbide testing

2. Testing equilibrium relative humidity (ERH) using Hygro sleeves.

3. Testing equilibrium relative humidity (ERH) using a floor humidity box.

Since options 1 & 2 involve destructive testing, then the only option available for ongoing moisture tests in the flat is option number 3. A floor humidity box should be placed on the large repair section and sealed with double-sided Butyl tape. We recommend that the humidity level inside the box is then recorded after a 72 hour period. If the ERH reading is 75% or less then you are free to proceed with the installation of finished flooring. Manufacturers information on the calcium sulphate screed used for the repair section indicated that the screed could take up to 50 days to dry so clearly an initial period of drying would be required before monitoring was started.

Ongoing Tests

The developer chose to use a ‘Tramex Hygrohood’ floor humidity box sealed in place with double sided butyl tape for ongoing moisture measurement of the repaired floor section. However this proved what we already know, in that readings of <=75% ERH are extremely difficult to achieve in floor screeds. Four weeks later I was called back to reassess the flooring after the developer failed to get ERH readings below 77%. Also you must remember that that ERH readings are relative to temperature as well as moisture yet product information makes no reference to the temperature benchmark at which the ERH reading should be taken. We generally recommend that rooms are maintained at circa 20oC when screeds are being dried out and this is a general recommendation that is often found; it therefore makes sense to recommend that an ERH reading of <=75% should be obtained with a room temperature of 20oC. The Hygrohood shows a reading of 77% ERH at 21.5oC but if we drop the room temperature to 20oC then the ERH increased to circa 80%.

The client asked if we could test with calcium carbide again and it was simply a matter of understanding what depth the latex screed could be laid to. This allowed us to take a series of shallow drillings to obtain enough material for calcium carbide testing. The test again proved that the repaired floor section was now completely dry and the developer could move forward with installing the latex screed. The drilled areas will be made good by the latex screed.

All this proves that obtaining accurate moisture measurement in the field can be fraught with difficulty and pitfalls. Moreover, I think some of the guidance information is poor; particularly the guidance that an ERH of <=75% should be achieved, perhaps <=80% ERH would be more realistic because once that figure was achieved at a room temperature of 20oC the floor proved to be dry. You need to adopt a pragmatic approach in the field, understand the limitations of your test equipment and thoroughly review build specifications and manufacturers product information.

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  • Jim Ryce

    Very interesting and we’ve a similar problem. We took are core and tested returning 0.5% moisture content but hygrometers vary through the 58m2 house by 81-95rh. Temp only 12 degrees in the house as we need underfloor heating off to take readings. We did note rises as temperature falls. Is this normal ?

    • Joe Malone

      Hi Jim,

      One of the key problems you have when taking ERH readings is ensuring consistency in ambient temperatures. Ideally look to take readings with a ambient temperature (Ta) of 20 degrees C. What you have experienced is perfectly normal, as ambient temperatures drop then RH increases and obviously as temperatures increase then RH drops. There is no point in plotting ERH readings over time unless you fix the temperature at which ERH readings are taken. Personally I’m of the opinion that recommended ERH levels are too high because we’ve consistently proven that when relative readings are still high, we’ve proven the floor to be dry using calcium carbide.

      Regards. Joe Malone

  • It does not say a lot apart from the floor was not dry when the coverings were layed. (a simple plate test should do) Or there was a leak near the doors which the developer fixed or the windows were not in when the screed was layed.
    Liquid screeds are not the problem, The contractors and trades after are.
    It is a very simple process. Windows in,lay screed,brush latiance off after 36 hours, leave to dry whilst plater boarding and skimming,force dry if needed and then floor coverings and finishing. Job done
    Chris the carpenter !

    • Joe Malone

      Your comment lacks a little context Chris… can you elaborate on what you mean by “a simple plate test should do.”?

  • As liquid screed drys from the bottom up we know by leaving a dinner plate over night, if there is moisture then it is still wet, to us the simple methods work. we have never had any problems, we have just layed 250m2 of porcelain on the ground floor on liquid screed without any issues.
    80% of builders can’t build and most trades only care about their own.
    I have just moved in to a new build, the developer builds 50 units a year and I have 36 popped 600×600 tiles. on traditional sand and cement screed. Even after telling them the problem the time served site agent and tiler can not get their head around it.

    • Joe Malone

      I tend to agree that most problems are caused by workmanship as opposed to issues with the materials but your plate test is extremely crude Chris, a slight variation on the commonly used plastic sheet test used in the States. If it was this simple and effective then I wouldn’t see the problems I see on new build sites. The fundamental problem is that if the plate isn’t sealed to the floor then you can’t rule out moisture caused from high RH in the room. Can you imagine getting involved in a construction dispute where the clients solicitor asks you the question, “And how did you determine that the floor screed was dry before laying the finished flooring”? Your answer, oh, we put a plate on the floor. It simply isn’t a scientifically or academically verifiable technique. If you are proceeding with finished flooring on this basis then I’d hazard a guess that its only a matter of time before you do have problems; particularly if you’re using pumped calcium sulphate screeds.

    • John Clarke

      To be fair to you when I first came across anhydrite screeds maybe 12 years ago I was given similar advice about taping plastic to floor to look for moisture next day under face of plastic and even advised by screed company to just leave a newspaper on floor over night and check if damp next day and I got away with it for a few jobs and I rather feel I just got lucky using thick heavy stone that stayed in place even if debonded.
      But boy oh boy did I have a big learning curve with these screeds since ..
      So I got real lucky but some Tilers I knew were not so by following similar method . There have more failures of debonding tiles fixed on anhydride screeds than any other substrate I believe and this is due to ignorance and product knowledge . I feel very sorry for the tilers that have been sued and lost their livelihoods because of the lack of valid information supplied by screed companies . In our trade Chris news does travel through forums as I’m sure they do for your trade and I think the word has spread that great care is needed with this type of screed and screed companies now give much more tech info about these screeds .
      Anhydrite Screed’s have been around with very little problem in France and Germany for last 20 years but they know their business and give correct advice for installers thereafter . Difference here in U.K. is that some go getting screed company here picked up on so called anhydrite fast track screed and starting bring it in with no understanding at all of the product . Fast for them to lay but after that day a nightmare for contractor to dry out and thousands of failures at immeasurable cost .thrre is a method that I have devised using anhydrite adhesives , primers and decoupling membranes which I think covers all bases but the. Whole tiling process being sucessfull is totally dependent on the substrate being dry to suggested levels.
      If someone followed your advice they could lose their livelihood and I don’t think you would feel good. I have probably fixed 20 floors on top of anhydrite Screed’s and looked at as many failures on behalf of stone / tile companies with restorative works running in to hundreds of thousands of pounds because of ignorance or unfounded knowledge As you are submitting .

  • Kevin

    Hi Joe,

    We have renovated our house and according to the flooring contractor (we have already laid the floor tiles, he is laying an engineered wooden floor) our floor is too wet. He came with a Tramex, gave me some readings, I didn’t believe they could possible be correct, so he came back with a digital Tramex, reading still too high. To cut a very long story short, I have done my research into this and discovered as you say in this article that even Tramex state you can only get a relative reading and not an actual reading so the readings he is getting are by definition unreliable, even the manufacturer says so! He was looking for a 1.5% or lower reading and it was typically between 3% and 5%. The floor has been down a little over 1 year. So either there is a big problem or it must be dry by now. It was laid 1st August of last year, I’ve had the UFH on for 4.5 months so I have just purchased my own hair hygrometer to test the RH myself. I have become more ‘expert’ in 3hours on the internet than the flooring contractor who does it for his livelihood. However having read your article it appears that even my hygrometer test may not be ideal. Can’t do the destructive test I don’t think because of the UFH. Where do I go from here if my test is too high as it appears getting a 75% RH reading is very difficult to achieve in the real world?

    • Rob Beckett

      HI Joe,

      I have asimilar issue to the one above. I’ve had a screed laid in November last year, at a thickness of approx. 65mm, By my reckoning, this should have been dry by the end of March at the latest. [90 day total, 40 days =40mm & 50 days = final 25mm ]
      We are planning to put Karndean flooring down, but still have readings of 98 – 99%.
      i arranged for the builder to come and sand the top level of the floor off, as he had said that this seal could be holding moisture in.
      I’d quite like to be able to confirm quickly if it is dry, as I can’t believe that after 9 months it is not dry at all!.
      How can the calcium carbide test be performed when underfloor heating pipes are installed?



      • Joe Malone

        Rob, I’m losing count of the amount of people who are contacting me with this problem. Yes, you’ll have laitance on the floor that will need sanding off. If you find the sanding pads clog up then chances are that it is not yet dry. However, it should be dry by now unless you have an issue contributing to the floor moisture content such as leaking UFH pipes. The carbide test is easy to carry out where UFH is installed. I can locate the pipes using thermal imaging or simply do a series of shallow drillings. Technically, you are supposed to obtain a sample from depth but I’ve compared and contrasted results from deep and shallow drilling a number of times and there is never any difference.

  • Neil Mullan

    Hi Joe,
    Very interesting article and following comments. I built my own house 5 years ago now and used a liquid screed with underfloor heating. In broad terms I’m happy but I have a problem at my back door. Prevailing wind and rain mean that often some water will drip off the door when open or blow in when the children leave it open. Over the last few weeks some minor cracking to the tile grout has become more pronounced and one large tile has started to rock a little. If I lift the tiles in the porch and scrape / grind off any defective screed, without going to the depth of the pipes, is there an alternative product I could use to repair the top layer which would bond to the remaining anhydride screed and not crumble in the future if it were to get wet?

    • Joe Malone

      Hi Neil,

      The problem is that Calcium sulphate screeds do not like getting wet so you should concentrate on keeping the water out, but my major concern is why has the tile failed? Did you use a cement based adhesive when fixing the floor tiles to the anhydrite screed? If so then unless an isolation primer was used then you may have an issue with sulphate attack at the interface between the anhydrite screed and the cement based adhesive.


      • Neil Mullan

        The correct adhesive was used and the only problem area us immediately adjacent to the back door, which does get wet. I think I’ll need to lift and relay the few tiles and try and get a better seal between them and the door threshold.

  • As a liquid screed applicator, it’s all in the passage of information from the screeding contractor to the client……

    1) Laitence removal as soon as possible (should be offered in house)
    2) Turn on the ufh as soon as physically possible, we recommend Amptec electric boilers if no gas is available until later in the project and couple it with dehumidifiers running in parallel with the heat ensuring all windows and doors are sealed!
    3) If no ufh present, dehumidifiers as above!
    4) Tapped, clear bags to the floor isn’t science I agree, but you can get a good visualisation if there is still moisture in the floor after points 2&3 have been running for a week.
    5) last but not least, using the correct adhesive to fix the material to the floor is paramount!

    We built our offices this summer, high humidity levels were being read in the room before and after screeding but I followed by own advise and 3 weeks after laying the good stuff we successfully tiled over 100m2 of office space using tilmaster adhesives’ gypfix with no primer and they have remained in place with no “hollow” gaps when I test the floor with a coin!! During the week the heat and dehumidifiers were on we had the plasteres in skimming all the walls….

    It’s not rocket science how to dry the floors, a decent contractor should use the time they have saved on screed application vs traditional application and invest in temp boilers or contract the mechanical contractor to do it!!!

    That’s it on this matter in my humble opinion.


    Andy Owen

  • Andy Pick

    Hello all. Im currently preparing the floor in our newbuild to accept UFH and an anhydrite screed. So much conflicting information. I’ve had to put extra 25mm insulation under the 150mm that was delivered to reduce the screed depth to around 50mm ftom the 75mm on the drawings. The UFH supplier said to put expansion gaps at door thresholds between zones but the screeder said not needed???? Some of the insulation moves when walked on as it seems to be curved slightly so the depth varies from 45mm to 60mm. I wish it was a timber floor with radiators…….

  • Martin Cowell

    We had an anhydrite screed laid (depth varying between 50 and 70 mm) at the end of February 2016 with UFH which has basically been on since that time. Our tiler has been several times with a Tramex meter and is recording readings of 7 and 8. we are having Vinyl tiles and he says readings need to be less than 3. Part of the floor is dry but a good 2/3 is not and his readings have not gone down in the time he has been coming. In the area that is not dry the screed has a dark brown mottled surface which does look like a “crust” and is almost “shiney”. We have 4 sample Vinyl tiles and if those are left on the screed overnight then in the morning there is moisture on the surface of the screed but not in the area which is dry (presumably this is like the plate test?) The screed was never sanded after laying but the screeder came back 3 weeks ago to sand it but frankly the amount of material that came off the surface was negligible (we swept it up) and it has made virtually no impact on the “crust”. Since the sanding we had another tiler come and measure the RH. We turned off the UHF the night before and he put down 2 boxes in the morning. He came back the following morning. The UHF had been off all the time. One was under the island unit (but which does have heating under it) and where there does not appear to be this “crust”. This gave an RH reading of 73%. The other box which was in an area which has the “brown crust” gave a reading of 84%. We also showed him an area where we had had a Vinyl tile lying on the screed. It had moisture under it and he put a box on this. This gave a reading of 79% and rising although I appreciate that this was not a proper reading. We have no faith in the screeder. The local rep for the Screed provider came yesterday and advised that either we put down a DPM before tiling or we have the floor properly sanded which would mean using specialist equipment that would get the “crust” off. Of course that will be very messy with dust everywhere. Should we have a carbide test done before we do anything else? Any other advice?

    • Joe Malone

      Hi Martin,

      We carry a Tramex CME but only to show clients how inaccurate they are. The problem is that they will also read surface humidity as opposed to relative readings in the slab itself so if you have issues with high RH then relative readings are fairly useless. Similarly, you say you’ve taken readings from two humidity boxes on the floor but these readings are also meaningless unless you maintained a static ambient temperature of 20 degrees centigrade when the readings where taken; I assume you didn’t? The laitance (crust) should have been sanded off as soon as the floor was dry enough to do it; this omission was a critical oversight. My advice is that you have the floor carbide tested for moisture because you’re chasing your own tail with these relative methods of taking moisture readings.

  • Neil Henderson

    We had a liquid screed floor laid 10 days ago- 75mm over UFH. The building is weathertight, but no heating yet. When it was laid the individual laying it said that he was unhappy with the consistency of the screed, which appeared to be separating out of solution. We sealed up the building for 48 hours, then opened all doors/windows to ventilate during the day and shut at night. After 5 days there was still standing water in some areas. Now the top 2mm or so is still wet in about 50% of the area so that when you stand on it the screed sticks to your feet and leaves marks. The remainder of the floor has set hard so that you can walk without leaving marks. We now have electric blow heaters and industrial dehumidifier running with the building sealed but it is not doing much. What would you suggest?
    Many thanks,

    • Joe Malone

      Its hard to comment with such limited information Neil but it sounds like the water ratio in the pumped screed was way too high. Where did you get it from? Presumably it was pumped in from a delivery wagon, or was it mixed on site?

      • Neil Henderson

        It was pumped in from a lorry- the guy laying it was not happy at the time with the consistency.
        Is there a risk of major structural problems with the screed, or is it just going to be the surface layer which will need remedying?
        What would you suggest I do- Is it worth getting an independent surveyor to inspect it for advice?
        Many thanks,

  • Martin Cowell

    Further to my post dated 21 December 2016 I arranged for Joe Malone to come and do a site visit. He carried out a carbide test on our screed and was able to confirm that it was completely dry and that there was nothing to stop us putting down our final floor covering. That has now been done and we are delighted. Thank heavens we found Joe. After months of dealing with tilers who just relied on Tramex meters or RH tests but obviously had no idea what they were doing or talking about. The original screed sub-contractor who came and sanded the screed (after it had been down 9 months) which was a completely pointless exercise and created so much dust which permeated most of the house despite our best efforts. My wife and I spent hours moving all the furniture out and then covering our brand new kitchen units but of course dust still got everywhere. I cannot believe the ignorance and stupidity of these trades people who kept telling us we would just “have to wait until it dried out”. Had I not taken the initiative and contacted Joe I think we would have still been waiting “for the screed to dry” in 2018.. Thank you Joe your knowledge, professionalism and advice were first class.

    • Joe Malone

      Hi Martin, Thanks very much for the kind words and for taking the time to post. I’m pleased you’ve now got your floor finished. Regards. Joe Malone

  • Peter Stone

    Is it even possible to get a guide as to when is the best time to do a carbide test and where to find a service in the local area.

    We have an extension with plenty of windows making it impossible to get stable readings for instance one of the corners that is off the scale on a hand held device gave me a range of 42%@21.4 to 75%@23.8 over 24 hours having left the digital hygrometer box for 48hrs first. Our fitters seemed to think that temperature is taken into account by the box thought I don’t have a high level of confidence in them.

    Our screed was poured Feb 24th.

  • James Staff


    We had a Anhydrite screed laid in a brand new extension in November last year. We are looking to have Carndean flooring but down but we are still getting high readings on the Hygrometer 8 months later and the flooring company are saying there is nothing they can do. We have the top sanded off months ago and have used domestic de-humidifiers.
    We have two hygrometers, one is at one end where the sunlight comes in and this is reading around 66 which is great. The other one is at the other end away from the sun and has been in 3 different locations. It has consistently been reading between 82 – 85 but in the last two weeks has risen to 89 and now to 92! I can’t believe after nearly 9 months this section of the floor is not dry?
    Problem is our numpy builder put 120mm of the stuff down not knowing what it really was.
    Any tips or advice please?


  • Kev

    I work for a screed manufacturer/supplier. When I talk to householders I always advise that the optimum depth to install flowing screed is between 40mm and 50 mm. If you install at over 50 mm you are just wasting money and asking for extended drying times.
    Anhydrite screeds should always be sanded as early as possible, ideally in the first 7-10 days, to remove laitance which will slow drying and become progressively more difficult to remove.
    If you have under floor heating, you can use it to dry the screed 7 days after installation and the additional depth, (up to 65mm), becomes less important, But you’re still wasting money on screed you don’t need.
    Modified anhydrite screeds for use with UFH can now be installed as little as 35 mm deep with a 15 mm pipe in a domestic application.

    As suggested earlier the keys to a good experience are a good screed installer and a good flooring contractor.
    A good screed installer will do the following things:

    Advise you to have a weatherproof property before installation. Roof on, windows and doors in.

    Advise you on correct preparation- slip membrane, expansion foam perimeters etc., or carry out the prep as part of the service.

    Flow test EVERY load 0f screed when it arrives on site, using a clean flow board and flow cone. Ideallythe flow should be between 240mm-260mm, up to 280mm is acceptable. Flow can be adjusted by addition of water at site if the flow test is over 220mm but below 240mm.

    Dapple the screed as soon as possible after installation and certainly before the bleed water comes to the surface. Two passes at 90 degrees to each other are essential. The best screeders will have different lengths of dapple bars to ensure they can get into awkward corners.

    Advise you/your builder about sanding the screed or offer a sanding service

    Leave you an aftercare leaflet detailing drying procedures, sanding, UFH comissioning, primers and adhesives.

    Never install floor coverings on any screed with underfloor heating without running the system through a heating and cooling cycle beforehand.

    As with all things you get what you pay for. If a screeder is really cheap there will be a reason.

  • Kev D


    Interesting reading here, we have just had screed laid at 50mm, it doesn’t seem to be particularly level after the dappling, and some parts of the floor seem to be lower than others (can tell by the standing water in low parts, and dryer sections in the higher parts), not to mention the screed actually seems at this stage to be up to 10mm higher in some sections than it should be i.e. where the new floor meets the old floor, this is going to cause a BIG problem with the tiling as we want large square shiny tiles that will show up any unevenness in the floor.

    So my questions,

    1, how “perfect” should the level be (we were under the impression that it should be pretty spot on level across the whole floor which is approx 30sqm).

    2, if we do have these high spots (and indeed the screed seems to be higher than it should be) what are the options to rectify this .. would floor grinding be the way to go (if this is the case I will be looking for recompense to cover costs from the screed company)

    Thanks in advance

    • Joe Malone

      Hi Kev,

      The answer is to some degree dependent on your intentions for the floor. As you probably know, the pumped screed is not a finished floor and does not provide a durable wearing surface.A latex self levelling screed would need putting down to provide a more durable surface and generally would be needed if an engineered timber floor was going down. There are tolerances for floor levels, generally a maximum 4mm out of level per metre for floors up to 6m across, and maximum 25mm overall in any other case. How do you intend to finish the floor?

      • Kev D

        Hi Joe,

        Thanks for the quick reply, the floor will be finished with tiles, they will be shiny large square tiles throughout, from the existing room, through to the new extension (knocked the wall through to create a big room), just concerned that because we have the large floor tiles, I know they are difficult to lay and have a good finish if the floor is anything but level(ish), I will probably go the crack suppression membrane route laid onto the screed before the tiles are then laid.

        We do have wet underfloor heating in both the old room and the new extension under the new screed (the original screed in the old room is sand and cement.

        I measure the level today because it is safe to walk on, and some places it is 15mm out of level from the datum point on the old floor, it gets down to anywhere between zero and 6 to 7mm near where the 2 floors meet.

        Seems like a complete cock up to me … I even mentioned to the kid who was helping lay the screed that I thought it looked too high, this was before they dappled it, and he said dappling will reduce the hight .. so I just left them too it … mistake I think

  • All these questions and the advice given is incredibly important and interesting because we are about to lay underfloor heating with either anhdryte screed or (presumably) standard cement in an extension. I have a choice also to employ a tiler and go it alone or, appoint my architect to oversee it, who clearly understands at least some of the problems discussed.

    The builder and a technician have suggested that we lay standard (presumably) cement because it will dry out more quickly and because the area is not so large that anhydrite is needed but, to leave a gap at the doorway between the 2 new rooms (whichever type of covering is used) to allow for any tiny expansion occurrences subsequent to installation.

    What the builder has not pointed out is that the door dimensions to both rooms have been ordered, based on anhdryte installation; as a thicker layer of cement needs to be laid than for anhydrite we would end up with an unwanted step over the door threshold!

    What has also proved to be very important is the necessity of keeping my architect fully informed of any changes to agreed plans.

    A big thank you then is due to all the above contributors.

    • Joe Malone

      Hi Stefanie,

      Anhydrite screeds are generally chosen where underfloor heating is installed because it is much more thermally react than concrete, which means it heats up quicker. Builders have a habit of recommending whatever they understand and are comfortable with, and clearly your builder is not comfortable with anhydrite screeds. There is nothing wrong with anhydrite so long as the product and pitfalls are understood and the underfloor heating is properly commissioned in line with the recommended protocol. My advice is to stick with anhydrite but make sure you understand the installation and commissioning process. Additionally, do not mix it with incompatible products, such as a cement based latex screed, the finishing screed (if required) must also be calcium sulphate based. Regards. Joe Malone

  • H Price

    We had a kitchen/living extension built early this year, the anhydrite screed was laid 2nd week January. Approximately 65m2
    UFH was switched on approx 4-5 weeks after is was laid, The laitance was not removed, in fact, we had never heard of it until we started looking into reasons why the screed was not drying out. The surface does not appear to have a crust I have to say and there was a lot of traffic ie builders etc after about a week of laying the screed and a lot of dust!!
    To cut a long story short, we are at a stage now where we would like to have our 60×60 porcelain tiles laid. We have bought a nicobond Hygrometer, the readings are chaotic as our extension has a large glass lantern and 7 bifold doors, it lets in a lot of sunlight. If the sun shines on the meter, we are looking at readings around 55%-65%RH, sometimes less than that. At night and early morning the readings are>85% RH, mostly we get < 75% RH readings throughout the day. You can imagine our disappointment when the numbers start going up in the evening. I also must stress that the temperature of the room is very difficult to regulate as the weather is fantastic at the moment and the room is very hot during the day as all doors are closed!! We have had 4 dehumidifiers switched on for 5 days and have collected 50 pints of water in total so far. The hygrometer readings have not really changed since employing the dehumidifiers and collecting all of that water.
    Ive spoken to a very technical lady at Schluter about Ditra matting and also at Dural – dura base matting.
    Both said that the matting will let the floor breathe due to tiny vent holes within the matting so that if there is a small amount of residual water left in the substrate, then it is able to escape when tiled. I would love to believe this and tell the tiler to go ahead and start tiling. The next problem is the various advise we have had about sealants and fixatives. Someone actually told me thats all we need to do is just cement the tiles with Gypfix and job done. others say we need a sealer for gypsum based screeds applied in 3 different directions and the use a regular fixing cement to bond the tiles.
    Is my floor ever going to dry out? Does anyone actually know exactly what the correct procedure is for drying out a floor and a realistic estimate?
    One last question, I have read so many times that trying to sand one of these floors after a certain length of time is impossible as the so called laitance has been baked on with UFH, so if this is the case, what should we do?
    Should I hire the sander and dust collector from HSS at £510.00 for the weekend? Is there an alternative that we could try that is more cost effective, as I don't want to spend this amount if little or no difference is made.
    Many thanks

  • Peter Trimmer

    Interesting reading.
    I don’t knowif you are aware or not but Tramex published a pdf with updated info on comparative readings a while ago (if memory serves me correctly I’ve had it about a year) for the CMEX and the CME meters.
    Gives more clarification on what readings should equate to 0.5% and 0.3%cm.
    Shame the MRH3 isn’t calibrated for CS.

    • Joe Malone

      No I wasn’t Peter, but I’m fairly sure they won’t be reliable. Tramex equipment is very good but I just don’t think that CS floor screeds can be accurately tested using relative methods.

  • Colin Williams

    I’ve just had an anhydride floor laid today and came across this website and am now worried!

    The room is 6m x 5m and the maximum depth of the anhydride is 38mm. It’s been laid on deep insulation and around the edges some ‘foam’ to allow for expansion.

    There’s no hurry for the builder as he can’t do a lot now until the bifold doors etc arrive in about 6/8 weeks. The main reason he suggested this was to have a very smooth finish for the Amtico floor we plan to have laid in probably late September. (It was a £400 extra)

    The company that laid it said that the floor will need to have an abrasive finish before the floor is laid by using a diamond grinding disc or similar as the primers need to penetrate.

    Any advice or help welcome as a company I contacted about laying the Amtico today replied that they won’t work with anhydride floors and he had a 9.75 on Checkatrade with lots of reviews!!

  • Alan Keeney

    Hi Joe.
    We had a calcium sulphate ( Tarmac topflow screed A anhydride ) poured in our new build a year and a half ago. After months waiting on it drying ( meter readings of 83%) we called in a screed manufacture who told us the floor need sanded and the crust removed. I hired an industrial sander and set about sanding the entire floor smooth. We then had the meters put back down and we got a reading on three different rooms in the house of 65% the tiler passed the floor dry and started to lay our karendien floor, it was the tilers first Experian’s of calcium sulphite screed. They used 2layers of primer flowed by a Gypsum floor smoothing compound they then putt 2more layers of primer on top then they applied a cement based product ( 300 something) it was all done 110sqm in 48hours.
    After four months our floor started to lift. The strange thing is there was a lot of moisture under the Karendean , however we have since discovered that although most of the floor has not lifted its evident that the whole floor is not stuck to the floor as the 300 product has not stuck to the primer. In essence we have a floating floor. The floor company are saying they did not do anything wrong and said the moisture was the problem . I then spoke to Tarmac about this and they said if the moisture came from the sub floor then it would have affected the primers as well as the 300 cement based product, they category ruled out the moisture coming up from the subfloor . Now we have had another company in to have a look at the floor for us, their opinion was that A the primer was far too week and had been watered down too much. They said they could tell this by the colour . Further they said that they would never use the cement based 300 product on top of a gypsum based compound as it can cause a reaction and fail. This appears to be what has happened . My problem is that the manufacture is saying you can use the cement based 300 on. Top of the gypsum. But the other company says that this manufacture changes position like the wind . But they are adamant that it’s a big no no to use cement of gypsum . I could take several more paragraphs on the politics that I think is involved. Bottom line it looks like we will need to go down the legal root to get this resolved. Your thoughts would be very much appreciated.

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