I am thinking about putting in a plywood floor and some sidewalls in the attic for some crude, but useable storage space. The ceiling joists are 2x6 fir at 16" o.c. with spans of 14' and 10'. Should I sister another 2x6 to the existing 14' long ceiling joists for the additional load, or will what's there work okay. I have an old Arch design book that says 2x6 ceiling joists spaced at 16" o.c. for attics with minimal storage can span about 13'. I don't know what "minimal storage" means load wise. Also how do you suggest keeping the temperature in the moderate zone during Midwest summers. Don't really want to run central air in the space. Will an attic fan placed at one of the gable ends work. Total storage area will be center 10' x 38' long of a 34x38 attic.

I need to frame in the ceiling and I’m not sure where to begin. I do know that I want a tray ceiling to hide the ductwork, gas line, etc. The joists are manufactured beams and the studs are 2x6. Can someone please point me to some pictures of a framed trey ceiling on the internet? I’d like to get a few different perspectives before I begin. Thank you.