The decision regarding the thickness of plywood to use for the subfloor in the attic has a lot to do with how you intend to use the attic and it also.
Best attic floor board.
To floor an attic you ll need to install a sub floor for support before laying your plywood floor.
What size plywood for an attic floor.
Installing attic flooring for light storage only.
Use old boards that are close to the same thickness in a given section of the attic however.
You ll have to construct a wooden frame or sub floor with boards 16 inches apart to provide extra support for the floor.
If your intent is to use the attic for a purpose other than storage you will need to consider a more finished look.
Use 1 foot wide boards in pine or spruce to make quick work of putting down a floor.
You don t want lots of different floor heights or walking in the attic will be difficult.
Be cautioned though that even with insulation hardwood flooring will cause a lot of foot traffic noise to the floor below.
First you ll need to measure the area you want to floor so you can calculate how much wood you ll require.
Also if the attic insulation is thicker than your ceiling joists you can raise the height of the joists to above the insulation by attaching 2 3 s to their top edge before you attach the attic floor boards.