After a little period of being stuck and needing to refer to a couple of library books, my virtual framing project is back underway. I've got all the walls framed, and the main roof. I haven't quite figured out how I want the living room roof to be yet, but I'm leaning towards a low-sloped hip roof. I'll handle the gable ends next. I've been helped a lot by the blog of a guy named Jonathan who has built a similar house. (Here's his post on framing his roof.) His detailed pictures have helped me to visualize what I need to do.
The rafters and ridge beam turned out to be easy in SketchUp, but I wasn't sure how to handle the top of the walls. I went with a 2x4 on end, then another flat on top, with the loft 2x4 supports running in between the ones on end. (See the detail).
Unfortunately it's a little difficult to make anything out in the pictures now, with it being just a forest of 2x4s! I'm not sure it will be any better when I start putting plywood sheathing on, since it will then be a box of wood with a pointy top. Not very interesting.