KLF - throw either a couple of extra factory crossbars or a Thule rack and you can easily carry those long pieces of lumber, etc on the roof. Easy peasy.
Those are 16' long, some types of trim are only available in that length. I had to buy cedar lap siding the other day in that length. The factory rack has a max span of only about 44", no way it could manage something that long (safely). No way it could carry a few sheets of plywood or sheetrock.
I actually have my factory crossbars removed, they are worthless for carrying anything IMO.