Failed self levelling compound

It’s one of THE most important materials in the flooring industry. It’s one of THE most important components in a flooring project. Self levelling compound (SLC) can truly make or break any floor covering. In particular, a floor covering that has been glued down. If the self levelling compound fails, the floor fails, it’s that simple! 

This picture shows a completely debonded self levelling compound (Failed). Picture supplied by; who were not responsible for the failured self levelling compound in the picture.There are so many reasons why a self levelling compound may fail. When I refer to ‘fail’ or ‘failure’, I am referring to de-bonding, excessive cracking, disintegration. Let’s take a look at some of the reasons..

Poorly prepared sub-floor 

Throwing a slc onto a flaky crumbly concrete sub-floor is a certain prerequisite to failure. One of the golden rules in the flooring trade is to never try and stick anything to something that isn’t stuck. That’s a rather simplified statement and does have it’s exceptions but for the most part true all the same. The idea is to create a permanent bond between the sub-floor (concrete substrate) and slc. If the sub-floor isn’t stable to begin with, then it’s a 99.9% certainty that any self levelling applied on top of it will fail.

Solution considerations :

Grinding of the original sub-floor.

Correct cleaning of the sub-floor prior to applying a self levelling compound.

Assessment as to the type and quality of sub-floor i.e Sand/cement, Anhydrite, condition etc.

Choosing the correct self levelling compound i.e Compatibility with the sub-floor.


If the sub-floor you are installing a slc onto is wicking moisture from the earth beneath and your self levelling compound or even any primer used in some cases isn’t moisture tolerant, you can expect failure of the self levelling as the moisture starts to break it down. This is particularly the case should there be an impervious or semi impervious floor covering installed on top of the self levelling compound. Essentially trapping or slowing down the evaporation rate of moisture from the sub-floor respectively, therefore exacerbating the effects on the self levelling. In circumstances like these, it’s often the case the self levelling will de-bond from the sub-floor – failure.

Solution considerations :

Assessment of the moisture content of the original slab i.e. Using British Standard measuring methods under the Codes of Practice BS 8203, BS 5325 and BS 8201.

Assessment of the nature of the moisture content i.e. Constantly wicking moisture, moisture content still carried from the mixing stage.

Assessment as to the hydrostatic pressure of moisture.

Appropriate use of a liquid or sheet damp proof membrane where required.


Deflection, in the context of flooring, means vertical up and down movement. Many self levelling compounds are not designed to resist deflection. Particularly in situations where the sub-floor is of wood construction i.e. Suspended timber joists with a finishing layer of floorboards/chipboard/plyboard. Using the wrong type of self levelling compound will often result in complete failure. Even self levelling compounds that are designed to flex to absorb any deflection have there limits.

Solution considerations :

Assessment as to the amount of deflection present.

Where required and able, the amount of deflection should be decreased or completely removed.

Correct selection of an appropriate self levelling compound i.e. Latex based, fibre re-inforced etc.


The compression strength of a self levelling is rarely a consideration to the layman. However, in the case of say a vinyl type floor covering or a covering with low structurally integrity that relies heavily on the integrity of the substrate beneath, the compression strength is extremely important depending on the location and use of the floor. When I say ‘compression strength’ measured in N/mm2 (Newtons per square millimetre), I’m essentially talking about how much direct load a particular self levelling compound can take before it collapses. To give an example, a hospital floor may have large heavy chairs and beds rolled across it. Should the slc be too soft or in other words have a low compression strength when fully cured, this would lead to dents, grooves, tracks, and dips forming.

Solution considerations :

Correct assessment as to the usage of the floor i.e. Amount of foot traffic, type of traffic.

Correct selection of a self levelling compound with an acceptable compression strength.


It’s always best to look closely at the preparation of your sub-floor. This has always been and will always be one of THE most important pieces in the puzzle. Cut corners at this stage and any subsequent layers may well be doomed to fail! Cutting corners on things that aren’t going to be seen is enticing. However, in the world of flooring it can be the biggest mistake you’ll make.


© Copyright 2015 Wes, All rights Reserved. Written For: Fitmywoodfloor

I am a Pergo trained professional installer of 20 years. I've been up close and personal with lots of floors and have the knobbly knees to show for it...Should you have any questions or comments please feel free to add them below. Thanks for taking the time to call by and I hope the information you've found has given you some insight!........................................................................................................................................................................“When you click on links to various merchants on this site and make a purchase, this can result in the site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network,,,,,,, affiliate window network.” This statement is to comply with current internet regulations regarding transparency to consumers.

Posted in Problem floors and poor installations, Self levelling compounds, Sub-Floor preparation, Uncategorized, Underlays Tagged with: , , ,
  • David Grundy

    I have had a builder lay latex on top of an existing ( but fairly recent) screeded floor on a 100mm concrete slab. It has a polythene Dom below.
    Anyway the latex has popped up in places. It looks like it has expanded in relation to the slab! Any ideas. Have you seen it before?

  • Hi David,

    A couple of ideas :-

    Poor priming, if any. Resulting in de-bonding.

    Poorly prepared concrete, too dusty, perhaps with contaminants on the surface i.e. paint, plaster (Gypsum) etc. Resulting in de-bonding.

    The concrete slab has contracted slightly during the drying process. Latex applied too early. Resulting in de-bonding tenting in places??

    When you say “fairly recent”, how long has the slab been down exactly? Also, when was the latex applied? What latex is it i.e. Brand and product name?



  • Robert Harvey

    Hi Wes,

    Good article, thanks.

    I’ve just a quick question please. I’m having some electric under floor heating put in on Friday and I want to make sure the floor is okay.

    So base floor is solid level concrete, then it has then had 10mm insulation board siliconed down, the next step was to put the electric matting down and then a flexible self levelling compound before tiling. Does this sound okay to you?


  • Hi Rob,

    Everything sounds fine excluding the silicone. I’ve never seen or heard of that method of fixing insulation boards down. We’d normally use a tile adhesive applied with a full bed. I personally would never dream of using silicone. If you’ve been given the go ahead to do this by the manufacturer, then fair enough. If not, I’d strongly recommend you contact then to confirm.

    The flexible compound sounds fine, but do make sure it can be used to encapsulate electric ufh matting/wiring.



  • SJ Nicholson

    Hi there,
    I need some advice with my floor. I’ve recently added a kitchen extension and transformed the garage into two new rooms. My current joiner has laid fine flow 3000 onto the cement with a bonding agent and is happy to lay the new vinyl floor on top as he says there is a dpc under neath. However, I also got a second quote for installing the floor and they’ve taken moisture readings which came back at 83rh. Are the readings higher because of the moisture around? The property has been open with the extension being built for six months etc… And the cement is about four weeks old in the extension garage so maybe hasn’t fries fully? He wants to remove the fineflow 3000 and paste a liquid dpc and then self level… I’m unsure what to go with as they’re both conflicting…

    Hope you can help.

  • Hi,

    As the concrete is so new, the moisture readings are consistent with what would be expected. Typically, if the concrete has been exposed to the elements, this can also prolong drying.

    I’d strongly recommend you go with the proper floor installer. The shear fact that he has taken moisture readings pretty much says it all.

    Fine flow (to my knowledge and from research) should not be installed beneath a dpm. This is why your floor installer wishes to remove it as installing a dpm on top with the intention of suppressing the current moisture in the concrete will likely result in the fine flow 3000 blowing/debonding. Your installer doesn’t want to take that chance, which is quite understandable.

    If no dpm is installed, once a vinyl is installed on top, there is a high/potential likelyhood the fine flow 3000 will blow/debond as the moisture from the concrete builds up.

    Without a shadow of a doubt, I would again, strongly suggest you use the services of the floor installer. He is clearly conscientious and has the knowledge and understanding that will potentially save you a lot of money in the long term.

    I hope that’s of some help.



  • YC


    We had a layer of cracked/damaged Vinyl Composition tiles. Instead of removing and replacing everything, Self Leveling Cement was poured over the damaged tiles. It seems the incorrect primer was used as well. Now, we notice some hairline cracks with the Self Leveling Cement. We’re concerned that the flooring will fail if we install anything over the Self Leveling Cement.

    If we lay new VCT tiles on top of the SLC, what are some issues that we may see?

    Thanks for your help!

  • Hi YC,

    Slightly concerning that you’ve gone over previously damaged LVT. Typically, the rule of thumb and best course of action would be to remove. I appreciate you had your reasons. My statements are mainly aimed at other readers.

    “We’re concerned that the flooring will fail if we install anything over the Self Leveling Cement.”

    That’s an extremely difficult question to answer. A few hairline cracks aren’t always a sign that the compound will fail or has failed. However, they’re certainly cause for concern. It can happen if the compound has dried out too quick but at a speed that isn’t detrimental. In other cases rapid drying can cause the compound to shrink and go extremely brittle as well as debond. If the compound has debonded, it will sound hollow with a light tap with a hammer, metal object etc. It may even break up.

    “It seems the incorrect primer was used as well”

    That can certainly cause a problem. Any layer that fails will inevitably lead to subsequent layers failing.

    LVT/VCT like any material will expand and contract, in this case with temperature change. Granted, with this kind of product, the ratio of expansion and contraction will be minimal, but there all the same. When shear forces are applied during the expansion/contraction process, such forces will transfer through the adhesive and pull on the compound. Also general moving load (foot traffic) can quickly break the compound. If the bond is weak, failing or primed to fail, you could well see the VCT blow.

    What we’re really talking about here is a gamble. Obviously if the compound has already blown then it would be fool hardy to continue. If the compound has a bond (a firm sound and feel), you may get away with it.

    It will ultimately come down to your own judgement, loss to gain ratio, and basic assessment as to whether you wish to proceed.

    My immediate advice would be to rip up and start again. This is obviously a limited opinion as I’m clearly not see the floor first hand.

    I hope that’s of some help.



  • Elliot Idle

    Hi Wes, I wonder if you could give me some advice on my kitchen flooring problem.

    1930s semi
    We knocked down the wall between kitchen and dining room and built an extension out- meaning a large 35sqm room
    Flooring on dining a room and extension is all timber struts. But the old kitchen floor was the original concrete slab (presumably without a Dpm underneath)
    The builders said they thought the slab was fine
    Poured down some screed to level it out
    Dried for a couple of weeks.
    Then we had engineered wood flooring floated over the entire room

    Flooring was finished in early March
    Over the summer we started feeling a bulge in the wood by the entrance door, which is where the old kitchen slab starts.

    The bulge became huge.
    Got the builder back
    He cut up the board over the bulge.
    It was completely dry underneath
    Foam insulation was dry
    But the screed had made a huge, solid bulge.
    He hit it with a hammer and it cracked apart in pieces.
    When I knock th concrete slab directly below where the bulge was, it sounds hollow compared to othe other bits of the slab I can currently access.

    He’s now talking about
    *taking all the floor boards up that cover the old slab (annoyingly we can’t access the entire slab as we’ve made a utility which is tiled, over a quarter of it.)
    *Re-screeding to level.
    *putting a latex Dpm down- I think it’s called instamarc (apparently like the Ardex you talk about)

    Here’s my issue- do you think it’s just going to fail again?
    From reading your posts- surely all the screed we can access should be knocked out. DPM painted over the old concrete, and then screed over that?

    Sorry for the long post. I just don’t trust the builder at the moment…

  • Hi Elliot,

    Strange one. I’ve never heard of or seen a compound bulge months after the installation – not that I can remember anyway. Real head scratcher!

    Your builders new approach is sound, although, it is largely dependent on the condition of the concrete. Without a shadow of a doubt, the best approach would be to remove the original slab and start again, bringing this particular area up to modern specs. Not strictly always required and obviously dependent on budget.

    Two points I’d like to raise should you decide not to replace the original slab :-

    “Re-screeding to level”. Sometimes applying a smoothing compound can be referred to as ‘screeding’. If indeed, your builder is going to apply a smoothing compound (not a sand/cement screed), then the ‘Instarmac DPM IT’, make sure the compound is ‘moisture tolerant’. This will be made clear on the compounds data sheet, found at the manufacturers website. If a compound is used that isn’t moisture tolerant, once the DPM IT is applied over the top, the compound may well blow/debond, resulting in the failure of the DPM IT and subsequently putting you back to square one. This may not happen, but it’s just not worth taking the chance as moisture tolerant and non-moisture tolerant compounds are generally of a similar price.

    “Hollow sound”. It may well be worth having your builder address this. If the concrete is hollow now (assuming it isn’t loose), it may well move/shear in the future, again, potentially resulting in the failure of the DPM IT.

    Should you decide to go down the preferred route and start a fresh (New slab). Then you are correct, the best approach would be to screed (concrete), apply DPM IT, then a smoothing compound.

    However, there can sometimes be an issue with this order (For either method – leaving the existing concrete in place or for the new). That being the finish of the concrete. If it is ‘pitted’ (small pin holes), this can result in pin holing of the liquid DPM. This is because the small holes in the pitted concrete can trap air. Due to the viscosity of the DPM and quick skinning/part drying time, the air can get trapped before it has had time to expel, resulting in pin holing (small volcano like holes). Then you apply a smoothing compound that fills these holes, comes into contact with the concrete, and essentially bridges the dpm (Wicks). This is something you may never notice and may not have an effect on the new flooring, but again, best to avoid this to lower the probabilities of failure.

    If the concrete is pitted, it is sometimes advised to apply a moisture tolerant smoothing compound first (with the correct dilution of primer – often 7 parts primer to 1 part water, then a second application at 4:1 – dependent on the porosity of the concrete etc) to obtain a smooth pin hole free surface, then the DPM IT. You can then go straight of the DPM IT with your flooring underlay, obviously once dry.

    Hope that’s of some help. I think I’ve covered everything but if you have any further questions, feel free to ask away.



  • Elliot Idle

    Thank you Wes. Appreciate you taking the time to reply.

    Great point about the moisture tolerant smoothing compound. You’re right- I meant smoothing compound, not ‘screed’ when talking about the blown/ bulge in the floor.
    I’m not sure about the moisture levels in the original concrete, but having had a look at some of it, it has some fine cracks running through. And the smoothing compound was between 2-8mm in depth.

    I wish we’d now had the original slab taken out when building. I can’t see how we’ll be able to knock it out now, mainly because we’ve built over half of it- utility room and some kitchen cupboards…

    If we can’t take it out, would you recommend smashing out all the smoothing compound we can access, and then (apart from addressing the hollow concrete) do the re-levelling with moisture tolerant compound and then DPM over the top?

    Thanks again Wes

  • You’re very welcome Elliot.

    Personally, I’d want to see the previous compound removed and start a fresh (ish – if you’ll be leaving the original concrete in place).

    Providing the original concrete isn’t moving as a bulk, the cracks aren’t ‘live’ (moving), and it’s in good sound condition, then you’ll be fine to proceed with the compound then DPM approach. You’ll also want to lightly grind the surface of the concrete to remove any contaminants i.e. plaster, old compound residue etc. This will also prepare the surface ready for either the compound, or liquid DPM should you deem it in good enough condition to apply that first (which is typically the best approach if possible).

    The cracks should be chased to create a small ‘V’ groove. This should then be filled with an exterior grade repair mortar like ‘Ardex A46’ or similar. This is important to assure the integrity of the compound and in particular, the DPM.

    All the best with it Elliot.



  • Elliot Idle

    Brilliant, thanks Wes. You’ve been a great help.

  • Hi Laura,

    My sincere apologies for the delayed response.

    There is a problem with the compound mix or preparation prior to installing it. Do NOT install the parquet on to that compound. As the seasons come and go, your floor will expand and contract. This puts shear (Pulling) forces on the adhesive. If the compound is flaking/poorly bonded, your floor will lift in due course.



  • Renee Lapa Calzaretta

    Hi, we had a sub for of slc placed in our basement which will have vinyl floor on top. I was wondering if flaking and marbling is normal?

  • Renee Lapa Calzaretta

    Here is a picture of the marbling. ..Thanks in advance for any advice yiu can give!!

    ….we had a separate job done where slc was used for tile to go on top and the concre7looked very smooth and even, not flaky, no frothy bubbles when I’m a bit concerned about thst floor

    …thanjs, renee

  • Hi Renee,

    Apologies for the delay in getting back to you. We’ve been extremely busy.

    The slc in the picture has been installed poorly unfortunately. The colour deviation suggests an inconsistency in the mix ratio. As you mention and from your earlier picture, the integrity of the compound is extremely weak.

    Would this be a problem for an ‘overlay’ LVT? Hard to say, but certainly not a good starting point.

    Would this be a problem for a glued down LVT? Absolutely, yes.

    To confirm your comment of when your tiles were installed, yes, the surface should be smooth, even (flat), flake free, and consistent in colour (Which is a sign of integrity of the mix and the installer knowing what they are doing).