Cannot use "Create parts" feature for a shape edited floor in Revit

[dimitri.b]

Is there a way to create a floor with multiple finishes using "create parts" and have falls?

I notice that when you create a bathroom that requires a fall and a different material from other areas the create parts button grey's out. 

From the image below I would like the unisex toilet to be a different material and to have a fall.

I know you could create all separate floors and that would likely work but is there a way to create falls and use create parts???

[ToanDN]

Create separate floors or create parts from a flat floor then edit parts geometry to create slopes.