New cement screed wobbles on new underfloor heating and insulation

Good morning, questioner.
There have already been good answers to your question(s) in this forum.
Why does a "sanitary company," which is familiar with gas and water installations, lay screed?
Quote: "(...) sanitary company that laid 4cm insulation boards without leveling the hollows "
But that is not the currently pending question.
It was correct to contact the contractor who installed the trade (here: screed and substructure).
After the height leveling screed had dried, a load distribution should first have been applied, for example, made of HDF boards. Only then the insulation layer.
However, the horse has already bolted.
It is correct that a rocking screed slab is not proper and professional. The question remains when the screed will tear due to permanent bending processes.
One solution will be to inject under the carrier board, or in this case, the insulation layer.
The "rocking" area is a good indication of where the measure should be applied.
It is advised against opening/breaking the construction and relaying the screed there, as the underfloor heating would also require extensive repair due to the intervention.
I suggest injecting an expansion resin.
But caution!
This special work should be reserved exclusively for specialized companies that are experienced with the processing!!
Why?
Well, expansion resins can realign entire buildings (case study: Burg Linn in Krefeld); the force "located" behind it must be applied in a controlled manner.
Anyone who now thinks about using construction foam against the cavity may possibly cross boundaries.
Personally, I do not want to condemn this solution outright, but I certainly would not officially recommend it.
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It will certainly not reassure you, questioner, but the fact was that the preliminary work was not sufficiently checked for flatness and possibly for compressive strength (keyword load distribution) and/or was not executed properly.
Therefore, a so-called "sympathy factor" toward the responsible party must be pushed back.
Because that would mean taking full responsibility as the client (colloquially speaking).
Here, I can only wish you a steady hand in overcoming the solution!
Regards and good luck: KlaRa

Good morning, questioner. There have already been good answers to your question(s) in this forum. Why does a "sanitary company" that is familiar with gas and water installations lay screed? Quote: "(...) sanitary company that laid 4cm insulation boards without leveling the hollows " But that is not the question currently under discussion. It was correct to contact the contractor who installed the trade (here: screed and substructure). On the height leveling fill, a load distribution should also have been applied first after drying, for example made of HDF boards. Only then the insulation layer. Now the damage is done. It is correct that a rocking screed slab is not proper and professional. The question remains when the screed will break due to permanent bending processes. One solution will be to underpin the carrier plate, or in this case the insulation layer. The "rocking" area is a good indication of where to apply the measure. The solution to open/break up the construction and re-lay the screed there is not recommended, since the underfloor heating would then also require extensive repair due to the intervention. I suggest injecting an expanding resin. But caution! This special work should be reserved exclusively for special companies experienced in processing it!! Why? Well, expanding resins can realign entire buildings (practical case: Burg Linn in Krefeld), the force exerted must be dosed. Whoever now gets the idea to use construction foam against the cavity may possibly exceed limits. I personally do not want to completely condemn this solution, but I would of course not officially recommend it. ----------- It may not reassure you, questioner, but in fact it was so that the preceding work was not sufficiently checked and/or executed for flatness and possibly compressive strength (keyword load distribution). Therefore, a so-called "pity factor" towards the responsible party must be pushed back. Because that would mean taking full responsibility as the client (colloquially speaking). I can only wish you a lucky hand in solving this! Regards and good luck: KlaRa

Hello! The screed was poured by the screed layer, a subcontractor of the general contractor. The sanitary company (which we hired separately) laid insulation boards and then the underfloor heating over the lightweight concrete. The sanitary company wants to implement the resin solution, they have done this before. But so far this has not happened. In order not to damage the underfloor heating precisely at that spot, the screed must be removed again there (otherwise they do not know where to drill and risk hitting the pipes). How else should the resin be introduced? I have not read anything about load distribution so far; for example, this is also not described by the manufacturer of the lightweight concrete. It says there that the concrete can be covered directly, with wood boards, insulation, or whatever. At least that is how I remember it; I would have to read it again. Unfortunately, nothing has happened so far either. We have to be more insistent there. It is already very annoying anyway, as everything else worked wonderfully and now this.

The pipes can be quite easily found with a thermal imaging camera or thermal foil while the heating is running.

The pipes can be quite easily found with a thermal imaging camera or thermofilm while the heating is running.

Very interesting! I will bring that up. That would of course be really good. Because the screed drying is now almost finished, and tearing it up again, even if it is only 1 square meter, would be annoying and would cost time.

Nothing has happened so far. But two possible minimally invasive solutions were discussed together with the screed installer:

1. Pour water on the affected areas, weigh everything down for 2 days, and hope that the part then settles.
2. Cut the screed behind the affected area and hope that the part in front then settles.

Does something like this have a chance of success? I am very skeptical. The screed is absolutely level, I checked it with a 2m spirit level. There was no sagging in the classical sense, so nothing has risen at the corners. And where nothing is uneven, in my opinion nothing can settle either? And meanwhile the screed has dried through, the process ends tomorrow.

And meanwhile the screed has dried through, the program ends tomorrow.

I believe these are two different matters. According to our builder, the screed still has residual moisture even after the screed program. It may be "ready" for some types of flooring, but I know explicitly that they will still carry out moisture measurements here before laying the wooden floor. Because the screed really has to be completely dry for that.