How far apart should floor joist be

I don't know "code" on this, but I've always heard 16" for 2x10s or 24" for 2x12s.

Don't count this as gospel yet, though. Let's see what others say.

Joists can be spaced 12", 16" 19.2" and 24". The normal spacing is 16" o.c.

12" usually under kitchens and diningrooms, 16" standard for the rest of the structure. I
f you are using "I" joist they are usually spaced 19.2 " and floor truss sytems are usually spaced 24".

ski, are these spacings the same whether the joist is 2x10 or 2x12?

ski, are these spacings the same whether the joist is 2x10 or 2x12?


Yes. You can space a 2x6 24" oc but the span will be restricted.

Example: (dead load=10 psf) southern yellow pine #1 2x6 at 24" oc, the allowable span is 9'7".
A 2x12 southern yellow pine #1 2x12 at 24" oc, the allowable span is 19'6".

The IRC code book, page 81, table R502.3.1(1) list floor joist spans for common lumber species