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.

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