There are several ways to do this. Do you want to have the "defines slope" parameter available? Edit the floor's sketch, and while in sketch mode select the line where slope will begin and look at the green options bar. You'll have the "Defines Slope" parameter, check it and assign any slope angle.
Another way is to use slope arrows. Also in sketch mode go to the menu and use the "Slope arrow" tool. This is a great one since it lets you specify numerical level offsets at the head and tail of the arrow. Furthermore, the slope arrow doesn't need to be inside the sketch or even contained within the boundaries. For example you can have an irregular floor shape and just trace on the side the slope arrow as needed. Try it and you'll see amazing results.
There's a third way in which your floor finish layer can have a variable thickness. This parameter is found in the floor's type properties. Locate the finish row and check the box under "Variable". Then when you modify the sub elements, you'll achieve the slope only in those floor layers you need.
