As long as you have a truck to carry the stuff, this sounds like a good idea, but put the plywood in the bed, and not on the roof. What you want to avoid is having the leading edge of plywood catch the wind. Somewhere just past 30 mph the panel acts like a wing and wants to take off -- with well over a hundred pounds of lift! With thin, unsupported, sheets you'll get them flapping around at much slower speeds. If you control the front edge, the rest of the plywood travels fine. Put the heaviest plywood on top of the stack, and if you use weights, keep them to the front of the stack. Take along a friend to help you load the plywood, and to keep an eye on the load while you drive.

Happy building.

PGJ