REVIT 2008 Architecure and Structure have made leaps of progress in the way of Roof, Floor and Slab tools.
With the Draw Points, Modify Sub Element, Draw Split Line and Pick Supports tools you can specify the relative elevation of points or edges on a roof, floor or slab.
You must start with a flat roof, floor or slab and if it's a roof it cannot be attached to another roof but I'm here to tell you "it really works wonders on modeling flat sloped roofs".
You can however 'attach' walls to the top / underside of these warped roofs and floors. You can pick 'beam supports' as shape criteria and the floor, roof, slab will form a split line over the beam and create the slopes. The beam supports themselves can be sloped!
(click the link above to play the CADclip or right click and "save target as..." to download)