B+ motorhomes

burlmart - The roof has a 1-1/2" x 1-1/2" square aluminum tube frame sandwiched between the ceiling and roof panels. This frame is around the perimeter. The interior space has about six (6) 1" x 1-1/2" square aluminum tube ribs - welded across. There are a couple front to back ribs on either side of the vent openings. The space between is filled with a material that looks like poly-urethane foam insulation. There are plastic conduit runs through the tubes for the pulling of electrical wires. I have crawled around on my roof several times. I tend to want to spread the load rather than placing my entire weight in a couple spots.