Expert: Dan Griffin - 4/12/2010

Question
I'm building a shed 18x18. Want to add 2nd floor to store more "junk". would use 3/4 T&G flooring.  will also  be using a garage door with 2x6 header.  I'm trying to figure out weather to use 2x12x18 for joists, or (maybe cheaper) build a girder of 3 2x12x18 set back about 10ft, then run 2x8 joists on top running front to back with front over garage door.  Would using the girder hold the floor weight?  would 3 2x12 be overkill?  use say 3 - 2x10s

Answer
Kurt, I think you will like the 2x12 joists better to avoid the loss of height caused by the girder/beam carrying the lighter joists.  The girder can't make the span without one or more posts  and their required footings which may also influence your decision.

Any calculation of capacity must begin with some idea of loads to be imposed.  Often times storage, especially paper and books, head toward 100#/SF loading.  Using 40#/SF live loading which would be similar to your living room would mean the girder/beam would need to carry about 350#/lineal foot.  Here is a very simple beam sizing table for a wood deck that will probably make 30-40# loading:
http://www.decks.com/article40.aspx

I don't know if your garage header is carrying any roof or joist load.  If the garage door is for a 7 foot door, it will need at least 2x8 header just for the doorway.  If there are additional loads above, the header will need to be larger.  If this is a 2 car door, 2x12 will sag out on their own over time even in a gable end (usually non load bearing) usage.