I transported 2 sheets of 6MM and 3 sheets of 4MM Okoume 220 miles on top of my Jeep Grand Cheroke with roof racks. I had a piece of 1/4 inch hard board on the bottom. I duck taped the four courners and the leading edge of the pile all together to avoid seperating the pile. I used 2- 2x4x8's to stiffen it and after wrapping it all in a blue tarp (it looked like rain). Never drove over 58 miles an hour. The tie down straps howled above that speed. Worst part is the wind blast from the 18 wheeleers flying by.