Hi Andariza,
If you model the roof initially using the mesh tool, you can convert it to actual roof segments afterwards by using the Mesh-to-Roof add on. You can then adjust the roof segments afterwards to be closer to what you want.
If you don't have this add-on already, go to the help menu in ArchiCAD and look for the 'ArchiCAD 13 Downloads' link which will open the download page. Follow the instructions in the help file for installation and usage tips.
I can't think of a fast way of modelling the structure to support the roof. I 've got a feeling you will have to manually draw beams on plan using the nodes of the mesh/roof to snap to, then adjust the height of each end individually in the 3D window.
Hope that helps.