As an instructor, often I have to carry several boards with me to classes and events. Having a Subaru Forester, I can only safely stack so many on top. But by using the kayak style 'stack racks' you can add a few more boards securely as the stacks holds one side in place. Using this method you instead wrap your ropes/cam straps around the stack pole then around the main rack bar ontop your car.
Yakima has 2 versions of the stacker:
Kayak Stacker, $109.
Here's the products by Yakima, Here.