Greetings & Welcome!
Roofs & roof racks present multiple problems. Vehicle roofs are not designed to walk on, and roof racks put all that weight on your rain gutters or other small mounting points. If all that weight was equally distributed across the whole roof, it would be a much better scenario.
Typically people doing roof decks use a grid work of contoured ribs to more equally distribute the weight.
Cheers!
"The less you have that CAN go wrong, the less you have that WILL go wrong!" ~Murphy