Yes, in order to install and safely use a cargo box, you need to have a roof rack attached to your car's roof. This can be a factory-installed roof rack or an aftermarket rack -- either way, you'll be covered when it comes to most types of cargo boxes. That all said, there are cargo bag options that don't require roof racks. If you want to use your roof for storage but don't want to install a rack, you can pick from one of the many soft-material cargo bags designed to be secured to your roof via straps that connect to your car's interior and are held in place by its doors.