I'm not sure I've understood you right, but there is a conoid object (cone segment lying down) in the basic forms folder in the library. You could use that and then cut the protruding end using SEO with a sloping roof (or, to get the edge curve) a custom profiled wall as a hidden operator.
The rest is basic Archicad walls and roofs, look in the Help file.
You might look into the mesh to roof tool. This is in the Design-->Design Extras menu. Once you create the roof though it becomes pretty hard to manage as you will have a ton of little slices. For your roof plan you will probably have to use a fill to cover up all the crazy line work. Anyone else have any ideas?
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