That's one of the mistakes they made with the current racks.

Each kayak rests on two steel bars covered with foam pipe insulation. This works OK for the fiberglass boats. But some of the cheaper boats are "rotoplast" construction - a plastic resembling polyethylene. This allows for an inexpensive boat, with a tradeoff of extra weight. These boats have to be stored upside down because if left resting on their hulls in the heat the bottoms become deformed where they rest on the cross bars. When resting on their decks they don't deform because the raised cockpit cowling adds rigidity.

tanstaafl.