A kayak carrier, or "J cradle" for your vehicle is the best method of transporting a kayak. It should be lashed down at each crossbar, as well as at the bow and stern to each end of the vehicle. Kayak cradles are recommended for boats being transported "flat" to lessen chances of deformation from being lashed too tightly to the bars.
Rotomolded kayaks can be transported on their edge or upside down (hull up) safely using kayak stackers. For composite kayaks we recommend that they be transported on their bottom using cradles to prevent deformation. Be careful not to over tighten straps as this could cause deformation in your boat.
Always check your tie down rigging often as ropes and straps may loosen in transport.
FOAM BLOCK RACKS
Foam block racks can be used for transporting kayaks if extreme caution is practiced in rigging the load. They should be wide enough for adequate support. The boats must be securely attached to the top of the vehicle through the doors or windows and attached securely to both the front and back of the vehicle. If these tie down points at the front and back aren't available on your vehicle, foam block racks should NOT be used.
If traveling when inclement weather is threatening, position the kayak upside down if possible. A kayak heavy with water can become dangerous. Use a portage cover to seal the cockpit if an upside down position is not possible. Periodically check your straps to ensure they have not stretched while wet.