Sealing the base slab before the roof is installed

Even though I am already talking to myself, just to clarify, if he uses the appropriate material for the entire slab that he would use as a base when masonry, would it then be technically correct to build the partition walls on the completely sealed floor slab?

Not weldable again...
Under masonry sanded sealing membrane (e.g. G200DD) and on the board welding membrane (e.g. G200S4)

And how does he connect them then? What would be, from your point of view, the disadvantage if I build up? What can happen?

The welding sheet is then welded onto the overlapping parts... It can happen that the masonry presses into the welding sheet. For non-load-bearing walls, I would have no concerns.

Hmm, so not as problematic as I thought. I thought that was completely wrong. So if I let him do it like that, I shouldn't have to worry? The only thing that still surprises me is the timing, why does he want to seal it already now, the roof truss is only coming in week 48. What is worse, water on the raw concrete slab or water on the [Schweißbahn]?

Bad is not both, very bad would be water between the [Schweißbahn] and the floor slab!