how to carry 20ft lumbers to home
Do NOT stick a board out your window!

The board will be angled outward, making your overall vehicle significantly wider than usual. Worse, you will not be able to judge how far it sticks out. Misjudge and someone gets killed.

Why not simply put it on your roof and tie it down to the front and back bumpers (or better, the tow points under the bumpers). Any extension past the rear bumper must be highlighted with a red warning flag attached to the rear end.

Also consider renting a truck from the big box store which charges a flat rate for a few hours.