A little bit off subject, but still appropriate.  I have a herniated disc myself, so can relate to getting a kayak on and off the roof.  I drove about 8 hours one time, and managed to wreck my back in under 10 minutes lifting a 70 pound kayak off the roof, and that was with help.

I said enough was enough, and sold the thing and purchased a Sea Eagle inflatable.  Rolls up to about 18 x 36, and can be put on edge in the center section of the van or it will go on one of those cargo carriers that extend off the back of the van, which is what I'm planning to do.  They are a little bit slower than normal kayaks, but at 42 pounds very easily managed. They roll up to a very compact package, and let me tell you I kayak a lot more than I ever have!

I put a small, German-made Torqeedo electric motor on mine this past year, and it's the cat's meow.  Yeah, a bit off the subject of roof racks, but it is a nice alternative to an expensive roof rack and the trouble to get a kayak or canoe up there.

Troy