Ask Andy “Rocking insulation and my screed has cracked”

I am Andy Parking Managing Director of the Multi Award Winning Speed Screed thank you very much for your question Matt.

I am very sorry to hear that you are experiencing some problems with your screed and your question is:

“I have recently had a screed laid and at the time the builders laid it I told them that they had not laid the insulation well as it was rocking in many places. They assured me this was not an issue. However now the screed has been laid and is roughly 6 months later, the screed is cracking all over the place. Should I be concerned and how would I fix?”

Watch Andy’s video to find out his answer:

There is an adage in the screeding industry, that the screed is only going to be as good as the substrate that it is going to be laid onto.

If it is going on to concrete, and insulation, I think as you have identified that you have insulation that is rocking. Then that does not leave a suitable substrate for the screed. The screed is going to derive some of its strength especially flexural strength, because if you think of a screed, generally it has particularly good compressive strength but then when it becomes long and thin the flexural strength is not as strong as it is compressive strength.

Its ability to resist that kind of movement really depends on what the substrate is and the strength not only of the of the insulation. Insulation needs to be of a of a certain compressive strength itself because it will compress and if there is any movement any rocking, over time it could be that the screed ends up failing, because it is very thin.

Before the insulation is laid the substrate should be checked that it is clean, and any mortar snots, or anything that is on the surface needs to be removed and scraped.  It needs to be nice and clean, if there is a membrane going down on top of the substrate, concrete/beam and block, make sure the membrane is nicely smoothed so that you are not creating a multiple of folds where you could get some instability with the with the insulation.

If you have done that hopefully there should not be any points where the insulation could rock and if you have identified anything, take them out so either take them out.

If you have a raised area in the concrete it may be that you need to grind or use a repair mortar just to make it a flat surface that will accommodate the insulation.

Rocking is going to lead to instability and to potential failures and you have experienced exactly that.  The insulation should be laid very tight with butted joints, laid in rows that are then opposite, so the joints are not all lined up, alternate where the where the joints are, and good practice is to always tape the joints as well so that creates a more stable substrate.

I think your worst fears have come to fruition and you have an issue where the screed is cracking now.

It is difficult to say whether the screed should be taken out because you could repair those cracks, but if you have issues underneath, that may not be resolved.  You may be able to repair with an epoxy resin.  A low viscosity resin that you could pour into the cracks will certainly repair them.

If you have isolated areas of screed moving, you can stitch those cracks to add some stability, however if you have still got movement, then you have still got issues with the substrate and you know you are probably just storing up those problems.

Yes, you may repair them, and it may solve the problem for a short while but i think you are going to be revisiting those issues further down the line.  It is not just the screed but the floor coverings, if you are using sticking anything to the surface then it would cause problems.

I think you have really got to deal with the issues, it sounds like a classic, the substrate is not fit for purpose, and it could well be that it needs pulling out and addressing rather than just trying to “stick a sticky plaster over it” and just solve it for a short while until it fails again.

There are some repair solutions but whether it is the actual solution to your problem to when it is actually the substrate that is the problem. I am sure the screed is probably fine and is not a problem, it is the substrate that needs addressing and without removing the screed you cannot address the issue.

The substrate may have settled now, however that would best a best guess, and the only way to truly know what is going on is to remove the screed.  You could perhaps remove it in a selected area and then look at what is happening, and it would give you a better idea of what is happening elsewhere.

I hope that helps Matt we are here to assist and help with advice if you need repairs or screeding.  Good luck with the project.

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