Let's talk construction, not Archicad, for a minute:
Why is the roof thicker inside than out? Do you mean the structural members are thicker?
The reason I ask: I find that the roof tool is terrible for creating both roofs and fascias. Assiming we are talking about residential, I usually recommend that the roof tool is only used for the roof sheathing, using a profiled wall to create the soffit, fascia, and eaves, and a second "lower roof" to represent the rafters.
It's not ideal from an ease-of-creation standpoint, but it rocks for making construction documents that look right.
Tom Waltz