In response to Randjack - why would you put all the heavy stuff on the roof rack? It would be a great place for clothes/tents/sleeping bags,/etc... - put the heavy stuff in the trunk or cargo area (w/ the exception of the gas - I'd get a rear rack of some sort if I was planning on carrrying any). Quite often storage in a vehicle isn't about weight, but about bulk.
Also - 100lb for a roof rack? Many of the Thule & Yakima cargo racks weigh much less than 50 lbs. I'd do some more research.
Heck, even a simple waterproof roof cargo bag would work for hauling extra supplies - just plan your load better, and adjust your driving habits accordingly.
Sam