Miriam--sorry I didn't notice your final question. The framing is similar to the vent framing. I added an additional mahogany frame epoxy laminated between the spars to carry the load out toward the walls. The interior bulkhead cabinet frame will add additional support. Too late I realized I should have carried the mahogany completely out to the walls. My full weight on the center of the opening deflected the roof by less than an 1/8 inch. With the A/c weight spread out over a larger are (and less than 2/3 my weight), I haven't measured any deflection at all. My roof spars are 1-1/4 X 1-1/2 cypress, strong and insect and moisture resistant. I figure if my engineering is wrong, the roof should collapse only if I hit a rough bump while driving and not while I am camping.