Thanks for your reply.
Yes, the two gable ends are different in both height and pitch (hence creating the hyperbolic paraboloid). The structure is modelled by drawing each rafter after corresponding heights from elevation and plan, as individual polygons, which have then been extruded and placed in the right location in plan.
I think I have already tried your second approach where the roof was modelled as a roof face to the right pitch - however, due to the two different angles of the pitch at either end this isn't possible. Are you suggesting to use the "Roof face" under the AEC menu?