I would agree with FM....check your owner's manual or look at the rack itself. Just about all the roof racks I've ever seen are actually only rated for about 150 lbs.
Another option you may have depending on the year of your vehicle, is some of the automotive products Cabelas sells like the cargo area protectors for SUV's. We put one in my fiancee's Trailblazer, and it has a 2" lip all the way around it, so liquids can't go all over the place and the prices are reasonable too.