How to create complex curved roof

I'm designing a building in Revit that consists of tapered rectangles, with each wall curving into a flat roof. I've made foam models of it but have no idea how to go about modelling those curves coming in from different angles.

Any help would be much appreciated!

The best way to do this would be to create a mass in the shape of the roof you want and then create a roof-by-face and select the faces of the mass which you want the roof to follow.