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
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.
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
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