I don't understand why you can't do this with a foundation slab but you can with a floor. They are pretty much the same thing. There are many times when I have a slab on grade (which I consider a foundation slab) that needs to be sloped in multiple directions. You can't do this with the slope arrow or other slope tools, only modify sub elements works
The option to vary the thickness should also be present for foundation slabs. Though they need to change it for both floors and slabs so that you can vary the thickness even with a slope arrow, not just by modifying the sub elements.
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