Car and SUV roofs aren't flat, nor are the roof racks...so depending on how many sheets and the thickness and the weight it can be done.
You have to allow for the curve by spacing it up off the rack like previously posted with a frame. OR if it's only a few sheets and they will bend slightly to conform to the roof shape, put a soft material in a roll under the front and back. like a kid's pool toy or other foam.
What's really important is the strap/rope or tie down to the front of the car!
The air movement under the front at higher speeds will lift up the ply and send it off sailing down the road. It must be tight and secure, really pull on it and make knots so that they won't come loose. A ratchet tie down is best, but a secure hook on the front bumper will be hard to find, so you may have to get down underneath and find a cross member, OH WELL.
Side to side straps go under the door edge or in warm weather can go through the rear window openings. The rear tie down is best on a trailer hitch IF you have one. Again you will have to find a secure attachment point down under, watch for hot exhaust pipes and other parts that move under way.
I lost a whole bunch of 4 X 8 Melamine which is very slippery and wasn't tied down, from the bed of my pickup, when I accelerated too fast from a stop light in the inner city of Detroit. By the time I realized it the "locals" had been there and it was gone.