A couple 2x4 boards longways with a notched end to prevent sliding in an emergency stop.
Putting lumber on a subary roof rack.
If i were to need to carry that much regularly i would build a simple rack to make life easier.
Sans roof racks use a blanket.
A pair of 20 web ratchets used as a fail safe in case the board slides to the rear of the vehicle.
My car with 4 x8 mdf board on the roof rack.
A couple boards width ways with a notched end to prevent them from sliding left.
As someone else suggested putting a piece of scrap lumber infront of and behind the roof bars is a good way of keeping everything secure.
That way the tension on the strap will not only hold the lumber down but hold it all together as well.
My car with roof rack with cross bars installed.
When i say loop around i mean one full turn around the lumber.
It s better to use racks.
Second trip 16 2 x4 x6 boards on my roof rack.
In the case of a naked roof you should be looping the rope through the car through the doors not the windows and around the object on the roof and a towel or blanket should be used to protect the roof of your car from the object.
4 web ratchet straps in a pair of x formations to hold down the board to the cross bars.
I d use some ratchet straps and loop them around your lumber and tie it down to the rack.
Last resort if needing lots of whole sheets would be the roof rack.
Just keep in mind that blankets can mar your vehicle s paint and putting objects directly on the roof can dent it.
They re also adjustable very user friendly and best of all stand.
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.