Screed drying - Incorrect drying program set

30 degrees is still too low for functional heating. The maximum design temperature should be reached at least. Usually 35 degrees. 50 degrees is oldschool. During the subsequent readiness heating, it is about getting rid of the residual moisture, otherwise tiles and parquet won’t work (cm measurement). It’s always a gamble. Sometimes 1-2 more weeks of heating is not uncommon and should be planned as a buffer.

It would therefore be a disaster if we had to extend the program.

Self-made stress. There is no guarantee for readiness after x days.

Hello questioner. This is confusion to the power of 10. You write about a "drying program" and refer to an electrically powered (external) heating device. What is that supposed to be? It can (and will) promote the drying out of the screed, but here, as already pointed out by others, the "functional heating" comes first. On the one hand, it shows whether all components of the underfloor heating work and are tight; on the other hand, slow heating relieves the stresses in the screed caused by drying. ---------------- Confusion 2: "But I am concerned about the expansion." What is that supposed to mean? What do you fear is going to expand, when, and under what circumstances? ----------------- Confusion 3: "The installer is coming by tomorrow. (...) If I heat from 30 to 40 tomorrow and from Friday to Saturday to 50 and then leave it like that for 2 days until Monday, is that enough?" So the underfloor heating works again? "It would therefore be a catastrophe if we had to extend the program." I would be happy to take you with me on my expert visits to properties so that you can see in practice what is actually to be understood by a "catastrophe." Currently, in a very high-quality residential property, all fine stoneware tiles in all rooms have cracked due to improperly or not carried out drying measures of the heated screed, which you currently have pending. The owner lives there with his family, all fixtures have to be dismantled, as well as the floor tiles, the furniture stored temporarily, the owner’s family relocated to a hotel, and the resulting construction debris lifted over the house by a crane to be set up and taken to the debris container. These would be costs which – transferred to your case – would have to be paid from your own pocket. No, a time loss to avoid such damage cannot be called a "catastrophe." Ignorance and/or ignorance (which I absolutely do not want to imply about anyone here) certainly can! ---------------- What does "2% were reached" mean? Were those mass %, CM %, or a value from a device with a scale from 0 to 100? How and over which cross-section of the screed was measured by whom? Is the protocol with weighing, pressure build-up in the pressure bottle, and the residual moisture result available in writing? Construction practice is not as simple as it seems, unfortunately, as you can see. Regards: KlaRa

Hello . Confusion 1: The screed is dried under the underfloor heating. However, not with our heat pump but with another device that is connected to the circuit. So, not a construction dryer.

Confusion 2: by expansion I mean the movement/stress build-up and release in the screed. Is this achieved without 50 degrees? The heat pump will only run at 35 degrees later anyway. So is heating up to 50 degrees necessary?

Confusion 3: Explained in 1. I agree that it is not a real catastrophe. But it does throw everything into chaos for us.

The screed was broken open. Measured as you described with loading, pressure build-up in the pressure bottle. Result is not in writing. Measured by a professional woman from the family.

Our current plan tomorrow is 45 degrees. Saturday 50. Hold for 2 days, then down in 10 degree steps. The screed is already dry. The function of the pipes is tested then too, right?

Hello questioner. Perhaps I often assume too much technical information to be known to the questioners, which is why I would like to apologize for my perhaps "brash manner" in dealing with you. The stress buildup mentioned under point 2 is due to drying. No additional temperature exposure is required for this, e.g. from underfloor heating. Every screed dries even without heating (otherwise all other, non-heatable screeds would never become ready for covering). Another point to counteract misunderstandings: Functional heating is very often incorrectly understood as "covering readiness heating" even by architects and construction managers. This technical misunderstanding will cost a poor parquet layer around €750,000 in a very representative villa I recently inspected in Munich due to moisture damage that has occurred and the completely to be renewed "Versailles pattern parquet". The CM residual moisture measurements were carried out professionally, but far too early and (due to the size of the object) at too few points. Nothing against the expert who took measurements in your property!! But based on my many years of experience, I distrust every CM measurement that I have not conducted and evaluated myself. Nevertheless, much success with all further construction progress: KlaRa

Dear KlaRa,

this now interests me as well. After how many weeks (with heating of the underfloor heating) can one be sure (if one cannot fully trust the CM residual moisture measurement or the person conducting it) that the screed is ready for covering? (with a calcium sulfate flowing screed). How many weeks does it take on average without heating? Are there any indicators for this?

Hello "Wintersonne".
Your question appears simple at first glance, but many aspects need to be considered behind it.
I will first answer your question from a simplified perspective.
There are conventionally installed screeds, i.e. wet screeds, based on cement as well as calcium sulfate. The latter was previously called "Anhydrite screed".
Such screeds are installed in residential construction with nominal thicknesses of 45mm and, under favorable drying conditions on site, dry in about 6 weeks until ready for covering. This rule applied to screed thicknesses up to 60 mm. For each additional centimeter, the calculation must be squared, so for example with 80 mm (thus 2 cm additional thickness) we would assume 6 weeks + (2 cm squared = 4 weeks).
Calcium sulfate flow screeds (abbreviation: CAF) dry faster due to the curing mechanism differing from cement screeds, so the readiness for covering can be expected earlier.
However, this does not mean that the readiness for covering has actually been reached. This can or must be confirmed by residual moisture measurement (the CM method is customary in the trade).
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That was the simplified explanation.
Due to the demands of builders and planners for the shortest possible drying phase of the screeds, the industry has reacted and offered so-called "screed additives" that, among other things, are intended to shorten the waiting time until readiness for covering.
Some of these work, others do not.
The "crux" is, however, that all screeds with additives not only deviate from DIN standards and thus must all be considered special screeds (a legal aspect sometimes also looms threateningly in the background), but the properties of the screed mortars have also changed as a result. Among other things, such screeds can no longer be reliably measured by the CM method in every case (and thus the readiness for covering cannot be checked).
With the calcium sulfate flow screeds you mentioned, it has become even more complicated.
It is not found in any literature, but we experts in the field notice that due to recipe changes made by manufacturers, the pore sizes in the hardened flow screed have become smaller. Which has a considerable (unfavorable) effect on drying.
To cut a long story short: Even with heated screeds, where logically the thermal pressure would suggest a shortened release time of the mixing water – the opposite has happened, instead a prolongation of the waiting time occurred.
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This means: The readiness for covering can only be determined by CM measurements or by a DARR test (this is only carried out by experts, not by a craft business).
According to current standards, a drying time of 5-6 weeks can be expected for cement screeds with thicknesses up to 55 mm, and 7 days for CAF (not as heated screed).
Provided that appropriate room climate is maintained (shock ventilation and external heating in winter).
I hope these (somewhat more extensive) information have shed light on the matter!
Regards: KlaRa