Paul,
Don't give it up. All of these programs take time to learn. You have several posts here asking about modeling and theaters are among the more complicated to model. Think about how the building is going to be built.
How much training have you had? Have you gone through the training videos? Is there an archicad user group near you? Is there another archicad user near you (UK as a location is a bit general)?
When I look at the roof, I see the ceiling, above it joists and above them the roof decking. If the roof is an plane, sloped or flat, you can make a custom profile. And/or you can model the elements separately e.g. the joists which you can use SEO's to subtract their volume from the roof composite...
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"