Floor Joist Beam Spans

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Typically, yes. I request kiln dried lumber when available, but it's rare to find kiln dried around here for anything larger than a 2x8 or so.

Taller joists usually result in a stronger floor.

A much shorter span rating, and you have to use thicker plywood to span the 24" joist spacing.

A 20' wide building with a 2x6 supporting wall at each end would result in a 19'-1" free span. That leaves several options:
Assuming a 30 Live/10 Dead (sleeping areas/attics)
2x10 at 12" OC = 19'-10" span
2x12 at 16" OC = 21'-1" span
Bump the load rating to 40 Live/10 Dead (all rooms)
2x12 at 12" OC = 21'-9" span
On the other hand, if the attic isn't tall enough to walk through and we're talking limited light storage, you could size the "floor" as CEILING joists.
Assuming a 20 Live/10 Dead (Drywall, No rooms, Limited storage)
2x10 at 16" OC = 20'-9" span
2x10 at 12" OC = 23'-11" span
2x12 at 16" OC = 24'-4" span
Obviously, it depends on what James plans to store up there, but I'd probably opt for 2x12's at 16" OC.
Anthony
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In a previous post James wrote...

Use engineered lumber I-joists. 14" TJI 210 @ 16" o/c should be about right.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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Good advise but if it is just storage as stated, I would go for a drop in the center as I described earlier and then not even be concerned about weight. Even stronger and a whole lot cheaper. Not to mention dropping from 14" to 6 or 8 gives a lot more head room.

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