Depending on whats on the back/underneath, I would plywood under (in the attic) and then fill the outside with the correct thickness of wood so it does not stick up.
Regarding the nails, make sure you dont use ones that are too long. I have made the mistake before and it makes it interesting when you bump your head when using the attic.
Regarding the nails, make sure you dont use ones that are too long. I have made the mistake before and it makes it interesting when you bump your head when using the attic.