You could use a 3D document:
1. select the roof you need
2. press F5 or menu view->elements in 3D view->show selection/marquee in 3D
you should have only that roof in 3D
3. make sure you are in axonometric view (ctrl-F3)
4. use the "look to perpendicular of clicked surface command" -either in toolbar or view->3D navigation extras
5. click the roof's top surface
you should now be looking at the real sloped surface
6. create a 3D document from this
7. use this 3D doc to place in layouts.
Hope this helps.
Best Regards.
