Thinking about this a little more there's another option that may be easier. Instead of using a surface or boundary splines you can define the shape and size of the roof using an X and a Y Spline. The X defines the bottom of the roof and the Y defines the pitch and curve. If the roof is not rectangular you can also apply an optional clipping spline to define the shape. The images below illustrate this setup:
I hope that helps, please let me know if you have any further questions.