When planning your rooftop packing, it's a good idea to know both the roof rack's weight limit, as well as your vehicle's. If you're packing a lot of stuff, it's not a bad idea to create a list with approximate weights to make sure you're in good stead. Heavy loads create more wind resistance, meaning lower gas mileage and a lot more noise. And exceeding the weight limit can cause damage to your vehicle, and possibly even a serious accident. SUVs have higher centers of gravity to begin with, so adding a heavy rooftop load could make one roll over.