Sealing the garage with a concrete floor?

There are lockable ventilation openings for garage doors. And ventilation grilles can also be provided in the [Attika]. This is the most effective ventilation. Depending on the roof design. A core drill hole in the wall, where a fan can be installed later, is also an option. But I would try it without a fan first.

In new housing developments, you see garages 90% of the time with the doors 10-20cm open...

Cheers to the precast concrete garage!

A garage is not a residential building! Therefore, waterproofing within the floor structure is not necessary. An 18cm thick screed? How is that supposed to work? Even with a 10cm thick cement screed, it is difficult to adequately compact the lower screed layer. More than 10cm, that cannot work. No screed installer would execute it that way. Furthermore, the question arises whether it should be a bonded screed or a screed on a separating layer. For a bonded screed, about 30mm to 50mm is common. An 18cm thick screed will crack when drying like a loaf of bread, which will be a delight to watch. My suggestion: 35mm cement screed bonded, no waterproofing, no intermediate foil, no slope. But instead, sufficient surface protection in the form of a sufficiently thick applied concrete paint. It must be ground beforehand, otherwise you can save yourself this step. Without grinding, it would last from 12:00 to noon!! The architect should ask someone who knows! Not necessarily in the "Gelben Seiten", but a screed specialist company with a lead person who has a master’s qualification. ------------------------------ Regards: KlaRa

An 18 cm thick screed? How is that supposed to work? Even with a 10 cm thick cement screed, it is difficult to properly compact the lower screed layer. Over 10 cm, that cannot work. No screed installer would execute it like that.

Furthermore, the question is whether it should be a bonded screed or a screed on a separating layer.

Unfortunately, I am not a screed specialist and have to rely on the offers we have received.

For the garage, all screed companies offered us:

Cement screed (according to DIN 18560):
CT - C20 - F4
Screed thickness: 45 - 185 mm

One screed provider additionally offered a layer of AKS mesh as extra reinforcement.
In the others, fibers are apparently already included in the screed mentioned above.

That you are not a screed specialist cannot be held against you. However, I, as an expert, deal with (among other things) this topic. So what could be more obvious than to take a look at my notes? There is no "additional reinforcement" for screeds. There is no reinforcement at all in screeds, as this term comes from structural engineering. And screeds are not constructions from a structural point of view. The chicken wire, often advertised as "reinforcement," is not prescribed as mandatory in any standard. And it has often caused more damage than was previously known. All bonded screeds over 50 - max. 60 mm are pointless and highly risky, as they are prone to cracking! So: do not act with half-knowledge on this subject (I do not mean you here), but get advice from those who know!!
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Regards: KlaRa

An 18cm thick screed cracks while drying like a loaf of bread, which will be a pleasure to see.
My suggestion:
35mm cement screed bonded, no sealing, no intermediate foil, no slope.

Thanks for your hints.
However, we want to have a level entrance into the garage from the utility room. The floor structure there is 18cm.

If we do it as you suggested (3.5cm cement screed), then unfortunately there will be a 14.5cm step to the house.

Additionally, we have empty conduits on the bare floor to the sockets in the garage, which we would not cover enough with 3.5cm screed, right?

How about adding insulation boards to gain height?