We took a little break from roofing, because it was becoming obvious the whole thing was a little wobbly. The sheathing -- OSB in this case, but you could use plywood, particle board, whatever -- is what keeps those stud walls from racking or becoming a rhombus. Stud walls are good for holding the roof up, but can't resist sideways forces... such as the ones I set up when I crawl around on the roof. Good thing I don't get seasick.
The coop is now solid as a rock, as any building should be. Above, you can see a cutout for the egg door. Below, you can see the opening for the people door and inside past it, the two tiers of nesting boxes.
Without a ceiling, nasty chicken-eating animals like racoons could climb right up through the attic and down into a smorgasbord of fine dining options. It was all hands on deck to get that installed, the kids helped hold it in place while I zipped screws through it into the truss chords. Good thing we ponied up the extra $2 for 1/4" plywood vs the 3/8" OSB -- the plywood is feather-light in comparison.