Tie down the boards lengthwise and widthwise similar to how you d secure a mattress.
Putting lumber on a subary roof rack.
A couple 2x4 boards longways with a notched end to prevent sliding in an emergency stop.
4 web ratchet straps in a pair of x formations to hold down the board to the cross bars.
Wood says side rails and crossbars are ideal but you can move whatever you you need to without them.
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.
Last resort if needing lots of whole sheets would be 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.
It s better to use racks.
A couple boards width ways with a notched end to prevent them from sliding left.
Sans roof racks use a blanket.
My car with roof rack with cross bars installed.
Just keep in mind that blankets can mar your vehicle s paint and putting objects directly on the roof can dent it.
My car with 4 x8 mdf board on the 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.
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.
One of the things that make the outback so versatile are its retractable roof cross bars.
If i were to need to carry that much regularly i would build a simple rack to make life easier.
That way the tension on the strap will not only hold the lumber down but hold it all together as well.
When i say loop around i mean one full turn around the lumber.