Use a tie down that wraps around the item and the rails.
How to secure something to a roof rack.
Tie on to a rack in two places then secure the bow and or stern via the hood system see long boat short car above and securing the stern with a line tied to d rings in the bed.
Two equally spaced straps running front to back and two side to side across the load will do the trick for most standard shaped items.
The pen and paper plus elastic example in the accepted answer and using a large 27 ratchet wrapped around the length of the load and secured to the rear and front tow hooks with thick protectors where it touches the car chassis to keep the front of the planks from flapping upwards and either snapping off or acting as a parachute.
On each side of the roof you ll see a rack that runs the length of the roof of the vehicle.
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.
Next pull the long end of the strap under the crossbar on the.
Ratchet straps are a cheap and easy way to ensure the highest level of safety when deciding how to secure items on a roof rack.
Tie down the boards lengthwise and widthwise similar to how you d secure a mattress.
First loop the strap under the roof rack s crossbar and drape the loose ends over your oblong load i e.
Wood said most roof rack systems for cars have a dynamic roof load of about 165 pounds.
I ll continue using my roof rack with the advised ratchet configuration to secure them ie.
On the front of the car it s not as easy to find one so you can get something called hood loops anchor loops.
Wrap lumber in a tarp to secure it and place a blanket down on the roof under the load to avoid scratching your vehicle s paint.
Leaving your rack on all year will increase the price you at the pump.
Roof racks have been known to fail at high speeds with lots of updraft so take the extra precaution of tying the roof rack down to the roof of the vehicle as well.
Remove your rack when not in use to avoid the added weight drag and noise.
What types of tie downs work best.