Joist structural question

[glw]

I am rennovating an attic for storage/study purposes (no permit) and currently there are 2x4s running along the floor which hold up the ceiling below. I have talked to several people and have gotten a wide variety of options for solidifying those joists so that they can be made to bear much more weight. Ideally, I would like to use the thinnest wood possible so that I don't lose attic height. One person told me I could sister in 1 or 2 more 2x4s to the existing 2x4s, nailing them close and often and get plenty of structual support. Another person disagrees and says I got to use 2x8s or even 2x10s. Total span is 14 feet.

DOes anyone know of a site which compares the rigidity of a 2x8 to 2-2x4s nailed together? Any other opinions on this?

thanks,
glw

[Wgoodrich]

There are a lot of IFs invoved in this quest of yours.

If this is an area such as a bedroom load [lightly loaded floor joists] then @ 30 pound load rate you must use a 2x8 @ 16" on center repetitive lumber span to clear span 14 feet.

Question is are there any "load bearing" walls below that ceiling that has "cement piers" in the crawl space allowing that wall to bear a load? If you have approved load bearing walls found under that 14 foot span then your span would be shortened because of that supporting wall below if approved to be a load bearing wall.

That is just a few of the IFs involved. If you repeat your span 16" in center. If you stretch that repetitve lumber span to 24" on center then you would have to use a 2 x 10 floor joist.

The above is for SPF spanning repetitive lumber.

Hope this helps

Wg