Like Emmanuel stated: try the roof generator plug-in.

I'm in the process of replacing ALL my roofs with ones generated by this plug-in. It works great and the resulting object size is way smaller than I had with using roof parts to compile a full roof.
You will have to learn how to handle it, especially with difficult forms, but the forum topic for this plug-in has many ideas and solutions explained. There's also a growing help item you can review.