Flooring in attic space
You need to install 'sleepers' on top of the floor joists. Install them perpendicular to the joists. If you install them @ 16" On Center, using 2 x 4's, you should be fine.
I suggest you DON'T install the sleepers to the floor joists! You should 'build' 4' x 8' sections and support them in a few 'areas' w/cut 2 x 4s. You don't want your 'temporary' floor to 'wobble'. You know - cross-brace the 2 x 4s to each other w/cut pieces. Also, don't 'attach' any of the sections to each other because, some day, you may have to MOVE/REMOVE them to do some work. Also, cut a good amount of 1" holes in the temporary floor for 'venting'. You can even leave a little 'space' between floor-sections for the same purpose. In short, the floor should be allowed to breath. ALSO, consider the weight on the joists in the attic that's going to be ADDED to by 1) the temporary flooring, and 2) the 'stuff' you're going to put on top of it. DO realize that depending on the joists, they MAY not be build/designed to hold much more weight. Consider this and spread out the weight. Put the heaviest 'stuff' over, or nearest, any weight-bearing wall below (NOT counting perimeter walls. These walls are most likely carrying the weight of the roof!) 5/8" plywood should be good.
My best to ya and hope this helps.
Jay J -Moderator
PS: Consider the height-loss you're going to experience. It will CERTAINLY cut down on the headroom !!!
PPS: God Bless America!