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.
How to secure loads on a roof rack.
What types of tie downs work best.
Another tie down material you should use is netting or a waterproof tarp.
Use a tie down that wraps around the item and the rails.
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.
You can use a bungee cord to attach items together but do not use a bungee cord by itself to secure your load to the roof rack.
On each side of the roof you ll see a rack that runs the length of the roof of the vehicle.