Download the floorplan from the link below and make the building in ArchiCAD. This is the Preston house from Stonewood Homes.
https://drive.google.com/open?id=1lnr94_UXd4ypf7pLyHUHZMy8smTwQs4Z
This video shows you two ways of adding a complex roof onto a set of walls.
Try each of the techniques described on your own set of walls.