Expert: Dan Griffin - 12/25/2009

Question
I have a 25x54 foot garage that the ceiling is used for storage and has 4 foot knee walls above the top plates, I would like to convert to a couple of bedrooms and intertainment area, the ceiling/floor has TGI 12 7/8" 16 on center spaning the 25 foot way, I would like to support the floor/ceiling to remove the deflection so a pool table and other things can be used, but the hard thing is i dont want posts in the garage. i was looking at LVL's tripled up between the TGI's and use some  kind of steel plates with threaded rod to secure a double LVL going the 54 lenght, any help would be greatful

Answer
Corey, you've been doing some heavy thinking about this one.  I don't think you have  TJI joists made by I-level as they do not have anything at 12 7/8.  You do need to find out exactly whose material you do have and what the span and load is for your system.  You indicate that you have some of this area decked for storage, but you want something stiffer.  I can't envision how you can have rafters  that would have knee walls in the attic that would line up with the double plates

I did not find a listing for for any brand using your dimension.  I-level won't span your distance with its 11 7/8 joist, so I suspect you are maxed out with dead load, drywall ceiling, and a few pounds of storage capacity.  I'm guessing that the garage doors are on one end of these joists with door headers carrying much of the load.  YOu really need a structural engineer to accound for all the loads being imposed including wind, shear, and snow loads.  Your center beam being carried by several 25 foot beams may be reasonable.  You don't have much depth to deal with the 25 foot span.  It might be more cost effective to double the existing joists to smaller centers, stiffen the existing joists with LVL "flitch" type plates.  Another idea would be to go to a deeper joist or flat truss to deal with the span and loose some of the height in the attic.  You need someone who can see the issues to help design a workable solution.