My car with 4 x8 mdf board on the roof rack.
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.
I d use some ratchet straps and loop them around your lumber and tie it down to the rack.
A couple boards width ways with a notched end to prevent them from sliding left.
Just keep in mind that blankets can mar your vehicle s paint and putting objects directly on the roof can dent it.
That way the tension on the strap will not only hold the lumber down but hold it all together as well.
4 web ratchet straps in a pair of x formations to hold down the board to the cross bars.
A couple 2x4 boards longways with a notched end to prevent sliding in an emergency stop.
Second trip 16 2 x4 x6 boards on my roof rack.
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.
One of the things that make the outback so versatile are its retractable roof cross bars.
My car with roof rack with cross bars installed.
When i say loop around i mean one full turn around the lumber.
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.
Sans roof racks use a blanket.
They re also adjustable very user friendly and best of all stand.
A pair of 20 web ratchets used as a fail safe in case the board slides to the rear of the vehicle.
Tie down the boards lengthwise and widthwise similar to how you d secure a mattress.
Wood says side rails and crossbars are ideal but you can move whatever you you need to without them.