Most of them. I recall a number of years ago Lance had an option for a walk on roof - they added a layer of plywood under the aluminum. I'm not sure if other brands had that option, but it would add considerable weight up high right where you don't want it. On my old Four Wheel popup I walked on the roof a couple times, but only when it was in the down position of course, and then staying on top of the framing members. Mostly I tried to stay off it and just reach things from ladders.