We're turning a 13×13 area of attic space into livable space. The new 13×13 area joins an existing room. The attic floor structure is: 2×8 floor joists and OSB board on top.
There is a ~5 inch step down from the current room into the attic space area.
Someone presented the idea of keeping the OSB board on top of the 2×8 joists and running 2×4 joists on top of the OSB board. The 2x4s would run opposite of the 2×8's and the OSB would be sandwiched in between the existing 2x8s and the new 2×4's. On top of the 2×4's, we would place two layers of plywood to bring it flush with the existing room.
Should the OSB be removed first?
Best Answer
You've asked one specific question here, and I'll start by answering that: No. Unless you have a particular reason to do so, do not bother removing the existing OSB.
From there I'd say your plan is reasonable. Running the 2x4s perpendicular should pose no problem, as they're supported every 2 feet. They'll span that with no detectable sag. Be sure to butt join them over joists.
Rather than two expensive layers of 3/4" OSB, though, I'd rip 2x6 lumber to the appropriate height so that you can use just one layer of t&g OSB as the new subfloor. This mitigates the overload risk, too.
Note: I assume that you've had the roof system inspected for use as a living space (or even for storage). With 2x8 bottom chords I assume that either 1) it's a very long span, or 2) it was designed for use as such. That's a critical point here.