I have a Thule rack and I've carried two 3/4" 4x8 sheets several times on it no problems. I just tie it down using one ratcheting tie down in the front and another in the back. If you try to lift or shove around the sheet goods with your hands, you move the car instead not the rack or the plywood. They've never shifted or anything for me. I just take it slow, and probably feel safer with that on my car then going out buying a hitch and trailer and having another 10 feet of vehicle I'm not used to.