Even with the roof rack that comes pre-installed on your new car the "rated capacity" is likely to be quite limited. If the "car" is big enough to actually carry useful loads, you'll probably need a ladder to reach the load (on the rack) to tie it down. Of course then you'll need a ladder to reach to tie on the ladder so you'll have it to reach the tiedowns to unload, and you'll need to borrow a ladder when you get there to get your ladder down so you can undo the tiedowns to unload.