Caution: If something is carried on top of the vehicle that is longer or wider than the luggage carrier -- like paneling, plywood, or a mattress-- the wind can catch it while the vehicle is being driven. This can cause a driver to lose control. The item being carried could be violently torn off, and this could cause a collision, and damage the vehicle. Items may be carried inside. Never carry something longer or wider than the luggage carrier on top of the vehicle.
If your vehicle has a luggage carrier, you can load things on top of your vehicle. The luggage carrier has side rails attached to the roof. You can get sliding crossrails through your dealer to use for tying things down. These let you load some things on top of your vehicle, as long as they are not wider or longer than the luggage carrier.