Jill has a point about the storage. Should have mentioned that.
#1 - do you have a place to store it at home (back yard, basement)? I keep mine in the basement.
#2 - If you don't want to drive around with the empty shell when you are at your destination or do you have another place to store it? I used to worry about theft so I brought it in and put it on our balcony.
Definitely pros and cons to weigh.
As far as your question, most of the hard carriers do not require a roof rack. We drove our first 3 trips in a sedan with the carrier on top. No rack, no problem. They have special rubber coated hitches that attach to the edge of the top of the doors (that may be hard to picture but it is simple).
Good luck.
(sorry about the double post, threeboysmom was using my puter. I thought I could delete the first post)