My son has a closet in his room that has a small space above it (about 22 inches between the top of the closet and the ceiling).  He wants to set up a fort up there.  

I can build a railing around the closet and a rope ladder to get up and down.  I’m not sure what to do about the “floor”.  Right now it is drywall over 2x4s 16″ oc.  I think I should do something up there to give it more strength.  Should I just add some sanded plywood or sheathing?  If so, how thick does it need to be?

Thanks in advance,

Mike