4 web ratchet straps in a pair of x formations to hold down the board to the cross bars.
Strapping lumber to roof rack.
Next secure the load to the rack using ratchet straps photo 2.
On each side of the roof you ll see a rack that runs the length of the roof of the vehicle.
My car with roof rack with cross bars installed.
Acquire ropes and straps.
Use a tie down that wraps around the item and the rails.
Second trip 16 2 x4 x6 boards on my roof rack.
That prevents the bundle from sliding forward backward or side to side.
My car with 4 x8 mdf board on the roof rack.
A pair of 20 web ratchets used as a fail safe in case the board slides to the rear of the vehicle.
Thread the strap end through the buckle and pull.
Start by loading dimensional lumber onto the rack with the narrow edges resting on the crossbars.
Tie the load to the roof racks with nylon straps that have self clinching buckles.
Wrap the strap around the load and under and around each rack.
With the third option crossbars with side rails you can secure an item to the side rails and run tie downs from front to back securing them to the crossbars.
The second option is the side racks.