Hello questioner.
In such technically complex matters, one should not rely on half-knowledge! Therefore, here is the professionally correct information:
"In our case, the screed was laid on 24.07. Tiling began on 20.08."
Answer:
After not even 4 weeks, the hope that the screed is ready for laying exists only if a fast screed or a calcium sulfate flowing screed was used.
Determining the readiness for laying (in this case the dryness) is the task of the floor or tile layer!
He must carry out this test according to the CM method, as a standard test!
If the underfloor heating was not yet in operation at the time of tiling, the risk of excessive residual moisture in the screed is very likely.
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"However, the tiles were laid with a decoupling membrane because we still cannot heat until now."
Answer:
A decoupling membrane has absolutely nothing to do with the residual moisture! It is used in critical, i.e., crack-prone screeds. In this case, it only made sense to expand the craftsmen's invoice by one item.
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"The tiler told us that we should switch on the (Note: the underfloor heating) only earliest 4 weeks after tiling, and then only step by step."
Answer:
Wrong and nonsensical!
First, a heated screed is warmed up, then a CM measurement is carried out, only after that (with sufficient drying) does tiling take place!
The heating is done, by the way, according to a specified protocol according to "Interface coordination in heated floor constructions," thus not vaguely formulated as "step by step"!
You can download the protocol at
bfse.de/assets/Uploads/buecher/BVF20Schnittstellen1.pdf
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"Also, I would like to know if it is bad that the screed was not dried by the underfloor heating?"
Answer:
Basically, screeds dry out regardless of the respective binder, even without heating.
With heated screeds, however, it is the case that the upper screed edge zone is usually (as it is in contact with the air) drier than the lower screed edge zone.
Moisture collects there and is only transported out of the screed structure after the second heating.
However, only if the screed surface allows this.
In your case, vapor-tight tiles were laid, which means a significant obstruction to drying.
Catching up on this now is no longer possible!
What will happen in the next 6 to 15 months?
If, at the time of tiling, there was still correspondingly high moisture in the screed, the screed will dry out very slowly afterwards.
But the joy gives way to the note that the drying-induced deformation of the screed, which takes place unnoticed without any covering, will now lead to significant warping of the screed slab(s), causing the tiles (since firmly glued) to bulge.
Due to the compressive stress (from use), the tiles will break! No joke.
This first shows in the form of fine cracks which will run across the entire surface.
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What is to be done?
Document everything!!!!!!
What happened or did not happen when in the construction process (such as the missing CM measurement).
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You may realize at this point that non-binding answers in a network do not always reveal what matters, what is truly important.
In any case, I wish you much success on the future path: KlaRa