This is actually a difficult exercise to explain but a great lesson to learn.
In the REVIT video Tutorial below we demonstrate how to create a flat tapeing floor (or roof) where the underside of the floor (or roof) stays horizontal.
As well, the challenge is the floor edge has an elevation at the perimeter and an elevation at the interior where there is a donut hole (rectangular).
We use an in-place family using the 'floor based' template and set the catagory to 'floors' and then use a solid extrusion and a void blend.
For the 'creme de la creme' we tie it all toether with a few grid lines for total control.
I should also point out that you could actually accomplish the same results by making the floor layer a 'variable' thickness and then use the floor / roof 'shape editing' tools to add in the low points.
(TIP: Try clicking the 'HD' button, THEN the full Screen button. . . . Youtube is still working on this feature)