Floor slab is damp on the underside - what could be the cause?

Hello, maybe someone has an idea or can reassure me. It’s about a spot (about 1m) on the base slab that has always remained damp after it rained (marked in red). The soil on site was immediately removed again, so it couldn’t have come from there, it was already there before.

Where it was dry for a longer time, this spot also disappeared. Before the house was set up last week, it was very rainy and the spot appeared again. Then when the house stood for a few days, the spot disappeared again. However, yesterday evening it rained heavily again and now a minimal damp spot is showing again, see photos. Also inside, at the cutouts for the pipes, it is damp under the slab, I saw that earlier. Where could that come from? (I didn’t check before yesterday’s rain if these spots were already dry)

The plumbing company is coming at the end of the week and the screed at the end of next week.

I have already spoken with the building surveyor, we have to check if water is coming from above, but he can’t really say much more now. But I almost doubt that, because the house is sealed now. Under the base slab there is a 120mm perimeter insulation with foil and a very thick gravel layer (almost 2m below clay soil) with 5cm of crushed stone, plus frost skirts. The base slab contractor said maybe residual moisture/condensation.

What do you think? Thanks in advance.

Floor slab damp on underside - what could be the cause?

So that future readers or even those willing to help, when your thread history gets longer, can still find the way to the exegesis of this situation, you would best link the previous threads here yourself - I'll catch up on that for you:
https://www.hausbau-forum.de/threads/pfusch-bei-erdarbeiten-baugrube-bis-ins-grundwasser.47815/
https://www.hausbau-forum.de/threads/geo-textilvlies-bei-gruendung-falsche-robustheitsklasse-grk.47837/
https://www.hausbau-forum.de/threads/drainage-laut-bodengutachten-aber-wie.47852/

By the way, a bit more explanation regarding the picture story here wouldn't hurt.

Hello, thank you very much, but I doubt that has anything to do with it. During the earthworks, it was clarified that everything is fine and the other two contributions have also resulted. The geotextile at this depth is negligible and the drainage has also become unnecessary in my case, as I opted for frost skirts.

Brief info on this contribution:

My slab builder says the water is harmless.

Hello,

was a foil inserted between the insulation and the concrete (slip layer)? At the cutouts of the pipes, foil can be seen. Water can also penetrate on the other side and flow between the base plate and the insulation up to this point.

What I find much more concerning are the cutouts themselves. The base plate is interrupted there. How is that supposed to be sealed? Normally, the pipes are cast in the base plate with appropriate end pieces.

Best regards

Hello, yes the foil was applied over the entire surface. Okay thanks, but the water from below wouldn’t be concerning, right? That’s also the statement from Glatthaar. From above it is now almost impossible, because as far as I have checked, everything is sealed in the house and the area around the slab is dry.

Yes, the cutouts were discussed again with my house builder and the floor slab builder and it is normal at this company. I have also just seen several slabs in the company’s house building forum that were executed this way. These cutouts will be specially sealed again.

Best regards and thanks.