You actually didn't have to add transparent layer below - one can create single layered composite structures (I've heard it is actually good practise, not sure why though, besides the cut pen control).
Just keep in mind to set layer type to Finish/Other to extend slab into a door opening.
I know I'm bringing back old topic, but perhaps someone will find that info useful.
Łukasz Pietraszko
ArchiCAD 26 PL, since AC20 | WIN 10 Home
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