Is it possible to glue floating parquet flooring afterwards?

Parquet layers learn their craft during an apprenticeship period of about 3.5 years, followed by their journeyman phase.
They learn what a proper subfloor is, what is necessary to firmly bond the mineral leveling compounds to the subfloor, and for long planks, they measure the wood moisture before gluing, as this material can expand significantly due to swelling pressure.
For the gluing, they also learn that the correct angle of the notched trowel is required, changing the notched blade when it is worn down, so that the correct amount of adhesive can be applied to lock the parquet boards in place.
All of this cannot be learned en passant from DIY literature!
That is why my well-meaning advice is: Leave such trades to the specialist company that can recognize the many individual "pitfalls" – in contrast to the DIYer.
Spending more money at the right place can actually be the cheaper way.
At this point, I only want to mention the case (which I handled back then) where the DIYer glued parquet firmly in his living room by himself but, for aesthetic reasons, did not leave an expansion gap.
The parquet adapted to the climatic room conditions (in one summer) and, due to swelling pressure, pushed the gable of the house on the exterior wall side 5 cm outward.
As a result, a structural engineer had to intervene. Although the floor was cut open, support measures had to be carried out from a structural point of view.
In the end, the builder could have traveled to the Maldives with his wife for the amount that was subsequently legally claimed against him during the time a parquet specialist company would have performed the work.
Not every trade belongs in amateur hands!
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Well-meant advice: KlaRa

Thank you for your explanation. But if you look at it that way, one should not do any jobs alone at home. There are tradespeople who have learned everything for everything.
I do understand what you want to tell me. That’s why I want to start in a small room first.

That’s why my well-intentioned advice: Leave such trades to the specialist company that can recognize the many individual "pitfalls" – unlike the DIYer.

The question for you … for the OP:

Do I need to apply leveling compound on the screed beforehand to really get a completely flat, level floor? Or is it also fine when gluing directly, since the boards are firmly glued.

:
Long planks react very sensitively to unevenness that exceeds limit values. The larger the format (this also applies to ceramic tiles), the higher the surface flatness of the substrate must be achieved.
Compensation by impact sound insulation material is neither sufficient nor professional. Because the tasks of impact sound insulation do not include height compensation.
But the assessment of whether sufficient flatness has already been achieved or still needs to be achieved must also be carried out professionally. It is not enough to proceed subjectively and without knowledge of the basis of what needs to be assessed. This was, among other things, what I meant by training as a parquet layer.
The field that must be observed is very extensive.
And every installer, whether coming from crafts or wanting to carry out the work as a do-it-yourselfer, is obligated to inform themselves about all technical rules.
To answer the last question correctly:
Old leveling compounds can (conversely: this is the case in most cases!!) lose their properties through use over the years.
This includes not only a broken bond (so it lies loose), but also that they no longer have their necessary strength.
The technical rules state that all old leveling compounds must be completely removed up to the screed surface when relaying is carried out.
This aspect is particularly important with parquet, as very large forces are generated by the wooden planks (due to climatic conditions) which "pull" on the substrate.
If the old leveling compound can no longer fulfill its tasks but the new planks are still laid on it, this can quite possibly lead to total damage of the (glued) floor.
And this is not just said casually, but—unfortunately—the practice that I occasionally have to assess for courts.
Therefore my advice is also to leave such trades to specialist companies, as this then ensures the security of warranty.
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Regards: KlaRa

:
Long planks in particular react very sensitively to borderline excessive unevenness. The larger the format (this also applies to ceramic tiles), the higher the surface flatness of the substrate must be.
Compensation through impact sound insulation material is neither sufficient nor professionally correct. Because the purpose of impact sound insulation is not height compensation.
But also the assessment of whether sufficient flatness has already been achieved or still needs to be achieved must be carried out professionally. It is not enough to proceed subjectively and without knowledge of the basis on which the assessment must be made. This is what I meant, among other things, by training as a parquet installer.
The field to be considered is very extensive.
And every installer, whether coming from the craft or wishing to carry out the work as a do-it-yourselfer, is obliged to inform themselves about all professional rules.
To answer the last question correctly:
Old leveling compounds can (conversely: this is the case in most cases!!) lose their properties through use over the years.
This includes not only a broken bond (so it is loose), but also that it no longer has its necessary strength.
The professional rules state that all old leveling compounds must be completely removed down to the screed surface when re-laying.
This aspect is particularly important with parquet because the wooden planks (due to climatic conditions) build up very large forces that "pull" on the substrate.
If the old leveling compound can no longer fulfill its tasks but the new planks are still laid, this can definitely lead to total damage of the (glued) floor.
And this is not just said casually, but - unfortunately - the practice that I have to assess from time to time for courts.
Therefore also my advice to leave such trades to specialist companies, as this then ensures warranty security.
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Regards: KlaRa

Hello KlaRa,
I would like to briefly chime in again. I don’t know exactly whether this is a general "recommendation" from you here or if it specifically refers to my case. We do not have old leveling compound under the parquet, nor was there any attempt to mask any unevenness with the help of impact sound insulation (except just a little bit to level the height at the transition).
Beforehand, most rooms had carpet. This was ripped out and the floors were sanded. There were no major unevennesses, but I assume that even the smallest unevennesses can cause problems here. It’s not like it creaks everywhere, only in some areas.
Maybe I’ll just leave it as it is for now; you get used to a lot.... ;)

Small unevenness should not be a problem. Raised spots (this can also include thick drops of wall paint) should be removed. Minimal height differences are not a problem. However, if the height differences are too large, you should work with leveling compound. A very long spirit level can help here. Your floorboards are very thick, so they hardly adapt to height differences and can quickly create hollow spots. With very thin floorboards, this problem does not occur. Otherwise, everything should work as you described. A problem could only arise if you cannot completely separate individual floorboards without damage, and then small damage becomes visible at the joints.

You can't learn all of that en passant from DIY literature!

Thanks for your explanation. But if you look at it that way, you shouldn't do any work alone at home at all.

That does not contradict each other. If you do it yourself, you must be prepared to pay some tuition fees if necessary or to make compromises in quality. You are doing that right now without complaining, so all good.
Respect for the skills of the experts is definitely appropriate. They exist among craftsmen and also among DIYers.