If you must haul your stack of 2x4s home rather than wait a day for delivery (which can also put you out $50 or so), lay the boards from front to back on the car roof to minimize wind resistance. Cushion the load with cardboard so you don't scratch your car's paint, and stack the wood neatly. Secure it at two or more points with racheting straps, wrapping the straps through your open front and rear car doors before closing the doors over them. Bypass bungee cords, which won't allow you to close the doors. Have an employee at the lumberyard check your work, and fasten a flag to it before you drive off—carefully.