I'm new and this and I just sort of figured this out. Sometimes when I do that it's a disaster, and sometimes it seems to work out. I can't remember all the measurements so I took some hasty measurements this morning. Here are some measurements and some pictures. We're in the process of painting it--this is the primer color and we haven't finished yet.
The first is from the back. The lower roof panels are 13 inches wide, the upper panels are 8 inches. You can see on the left, at the base of the upper roof, how it's open for venting (and screened) on one side and there is a piece of 1x tuff board trim to hold the left side lower roof in place. The lower roof panel slides under that piece of trim board
The second photo show the underside of the removable roof panels. The roof panels are screwed to those pieces of white tuff board, the roofing membrane is applied over them, and then when the roof is in place the lag screws keep the roof from sliding down. When you take the lag screws out, the roof will actually stay in place until you pull it down. The roof is made of two 9x8 panels which are screwed together, with a third smaller (12 inch?) piece added on the end to make for overhand on the ends
The last is a shot looking up from the front door. You can see the "rafters." They are nine inches from peak to bottom. They are screwed to the sidewalls, and then I added bracing in the form of 1x3's under the rafters--the pine boards you can see under the OSB. There are also braces going between the rafters, made of 1x pine. You can see one on the right side. It serves to keep the rafters plumb and also to keep the roof panels from sagging.
The side walls are 16 inches high, and the space from the track to the bottom of the "rafters" is approximately 10 inches. Th front door opening--the effective width--is 24.75 inches