Acquire ropes and straps.
Strapping lumber to roof rack.
Wrap the strap around the load and under and around each rack.
4 web ratchet straps in a pair of x formations to hold down the board to the cross bars.
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.
Tie the load to the roof racks with nylon straps that have self clinching buckles.
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.
Use a tie down that wraps around the item and the rails.
The second option is the side racks.
My car with roof rack with cross bars installed.
That prevents the bundle from sliding forward backward or side to side.
Then secure the front and back ends of each lumber bundle with ratchet straps to prevent movement photo 1.
Start by loading dimensional lumber onto the rack with the narrow edges resting on the crossbars.
On each side of the roof you ll see a rack that runs the length of the roof of the vehicle.
Thread the strap end through the buckle and pull.
Next secure the load to the rack using ratchet straps photo 2.