I'm sure that the roof-rack application is designed to take a decent load. I wouldn't want to stand on the sheet metal but a bit of flooring accross the roof-rack would be reasonable for standing on. Still I'd want to read the spec's for max load on the vehicle roof-rack before using any of these tents. For my money I'd rather store these tents on the roof while traveling and set them upon the ground when sleeping. The top of a car doesn't seem a stable platform to me.