Attic Subfloor Thickness
Joist dimensions are only part of the equation.
Attic subfloor thickness. Measure the plywood and attic door. Simple sheathing cdx or underlayment should work fine. There are several good t g underlayment brands out there but t g may be a little over kill for your application. It provides a stable surface for finished flooring material such as laminate.
Attic joists for live loads. Covering part or all of the ceiling joists in the attic is a cheap way to make use of wasted space. The difficulty comes from the precise measurement and cutting required and the labor of getting the sheets of underlayment in place. Installing the subfloor is the easy part.
How to build a subfloor in an attic. A subfloor is a stiff wood underlayment secured directly the the flooring joists. 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 depends on the spacing of the ceiling joists. You also need to ensure that the joist spacing is adequate.
Attic joists that are made from 2x8s may be acceptable for building your attic floor but because every room is variable there are no absolutes. For the small difference is cost use 3 4 inch sheathing or underlayment. Take the measurements for your subfloor and measure enough plywood so that you can cover the entire frame.