I'm not sure of the brand, or where to search on line, but there's a set of aluminum gutter mount roof rack feet manufactured, to which you can add your own custom 2x4 or other bars. They're pretty strong, and since you can make your own bars you can use something less slippery than the usual rack bars. I've seen them in fancy sporting goods outfits, I think, as well as on line, but can't seem to find them now.
I've carried pretty heavy batches of lumber just on a regular Thule rack, but it can flex a little. If you tie things down, and especially if you use a lot of duct tape to bind everything together, things don't move around much.
If you expect to do a lot of hauling of long things on a rack, do yourself a favor and go out and get some cleats, and screw them to the bottoms of your bumpers, and you can save a lot of trouble tying down and tightening.