Why UndergroundSlab HBSrf is recognized as groundFloors?

In the attached GH file, a HBSrf is created as UndergroundSlab using the createHBSrfs component.

However, it is recognized as a groundFloors surface rather than a undergroundSlabs surface using the Decompose Based on Type component.

May I ask why it is like this? What is the difference between groundFloors and undergroundSlabs surfaces?

Thanks!

ZJ_Tut_13_HBSurf_v001.gh (450 KB)

By definition undergroundSlab is a roof which is underground. A floor which touches the ground is a groundFloor

Thanks, Mostapha!

But I don’t quite understand what a “underground roof” is.

Is it a roof surface (with a normal pointing upward) whose centroid Z coordinate is smaller/“lower” than that for other “ground touching” floor surface, i.e. a “groundFloors” surface?

True! If yyou