Soft carriers are lighter, too. That's not such a bad thing!
So...I may start researching those.
They make them like HUGH duffle bags. You lower then from the top of the van and just roll them where you need them to go. It is a set of two, so by themselves, they likely wouldn't get too heavy unless you put books in them. Bulky, but lightweight stuff would work well, as would clothes.
More expensive...
http://www.walmart.com/catalog/product.gsp?product_id=2652148
Evening Rosie - you may want to check the owners manual for carrying capacity of your roof racks. Sometimes they have a metal tag or engraving that says something like 'roof rack capacity X lbs.'