Create a composite with skins for structure, earth etc.
Apply to what you are modeling the roof with, roof tool or slab. Parts of the assemblage that is sloped you will do using the roof tool. Parts that are flat can also be modeled with the roof tool with its slope set to 0° or with the slab tool.
Please add to your Profile your OS and also which version(s) of archicad you are using.
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"