Revit :: Creating A Shed Roof?

I'm trying to create a simple shed roof attached to the sloping walls of a rectangular building.  4 walls, south wall 10' h, north wall 11" h, and the east/west walls joining them are therefore sloped.
 
I want to attach a simple shed roof to the top.  Here's how I think I should do it:
 
1. create a new workplane aligned with the top of the walls.
 
2. switch to that workplane.
 
3. use "Roof" - Extrude" option to create the roof.
 
Assuming I'm correct in my steps above, I should be able to create the roof.
 
QUestion - How do I CREATE a new workplane where there is no object already to choose?  Program won't let me create a workplane in workplane viewer by picking the tops of the four walls (in any of the tiled views). What am I missing?