Hard to say without seeing more construction pics. I don't see any screws anywhere on the center of the roof panel, so I'm assuming it lacks joists/support on the center of the roof panel, which are important to keep the roof from sagging. In this case, I'd remove the existing roof, then attach 3-4 joists from that center horiontal 2x4, to the top of the outside coop wall - now you have pitched joists to attach the roof panel to. Fill in sides with lumber, or with HC to have a big vent across the top. Install the side door hinges onto the header in those openings instead of to the roof