Before I got Yakima kayak stackers, I carried my 17’ rotomold kayak upside down. In hot weather for longer trips, I would find the front deck above my legs had been compressed. Fortunately, it recovered it’s shape, but too long or too many times like that could lead to permanent deformation. Now I carry it on it’s side and even in very hot weather there’s no detectable deformation. I’ve also driven in heavy rain, and it drains out while on the side.