Hi Olivia,
My advice is don't use either the mesh tool or the roof tool, both of which look rubbish in section and aren't smooth enough in 3d, as you have found.
The tool to use is a brilliant library part called 'Mesh 3d' by petri, originally found in the object depository. This has some rendering issues but has been fixed by Olivier Dentain and the new version can now be found at the end of this thread here -
http://archicad-talk.graphisoft.com/viewtopic.php?p=105367&highlight=mesh#105367
Basically, you can bend all the vertices in any direction you like. Have a play with it to see actually what it does!