Revit sloped level.

[Elindio1]

I'm new to REVIT 2008 structural and I have the following question. How can you set a level slope. For example, a roof level with high point 21' and low point 16'.
I do not seem to find how to do it. Thanks

 

[ASCE07]

construct the supports at first. And then in 3D view draw the slab/roof...it will work.