I too would like help on adding the rakes to gable ends. I've done it a couple ways and keep thinking I'm definitely not seeing the better ways.
The first way...(and the gurus will think: how stupid is this guy??? But deadlines make one desparate) :Main roof by footprint/no overhang then an extruded roof with the desired cornice return off the end. I sketched the rake to leave a 1/4" shingle layer and did an in-place model for the shingles. I did this because I could not dedicate the top surface of the rake to shingles without the whole in-place model becoming shingled. Of course, there's a lot of line work required in the 3D and elevations.
The second way...left the main roof overhang at the gable and used a void extrusion to create the rake thickness. I fought hard with this to get it to render. Seemed quirky but it was quicker. I suppose if you do it right you can have the roof overhang just beyond the wall face and the void stop short to give you a below rake trim piece.
Revit Arch 2011