more questions about 24"OC framing

Started by cphillips, November 23, 2011, 01:39:29 PM

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We are building the 20x30 1.5 story house, with an additional 10 feet of length. The loft is over only the back 20 feet of the house.

We are framing the walls 24" OC instead of 16" for better energy efficiency.  Since the floor joists below the floor in the loft are also 24"OC, the county building inspectors are telling us that we need to use I-beams for the floor joists.  In our county, the maximum span for a single 2x12 (#1 Southern Yellow Pine) is 17 ft 8 inches.

Our builder thinks 2 2x12's would be sufficient for a 20 foot span, and cheaper than I-joists. Does anyone know that for sure and how can we prove it to our building inspectors? Any other creative solutions to this?


This is for the loft floor, right, not the main floor?

SYP #1 2x12 comes out for a max span of 19'6" with the AWC online calculator when using 30 PSF for the loft floor load. A loft is usually a sleeping area and can use 30 PSF instead of 40 PSF for a living space, for calculating. Ask what numbers they are using. If the span is measured from the wall edges that would make it, but barely. You might be okay with that and you might not. Pushing the limits is not always a good idea.

Doubled 2x12 would likely work, but as the IRC does not list/prescribe that as a choice the inspector may not go for it. Stepping outside the "code box" may require an engineer to agree and stamp the plans for the inspector to accept. Some are more versatile, open to non listed ideas, than others.
Just because something has been done and has not failed, doesn't mean it is good design.


Why do the joists have to run 24" O/C?  Why not run them 12" O/C.  It would pass code then.  I know a double 2x12 it is a little outside the box for prescription, but it is the same amount of wood carrying the load as 2x12's run 12" O/C.  That might be an argument you can take up with the inspector.

Edit:  Sorry, I didn't realize this was under plan support.  Please note, I am not affiliated with the site in any way.

John Raabe

Using the span calculator Don linked to a D/F select structural 2x12 @ 16" o/c will work. A number 1 grade will be plenty strong but give you a bit more deflection. If you go to 24" spacing on the joists you will need to go to an engineered joist or have a beam under the floor. Squirl's 12" o/c solution is a good one. It would let you use #2 or Btr 2x12 and only add a few sticks. Probably more economical than either the select structural or I-joists.
None of us are as smart as all of us.


Ok, thanks, everyone for your responses. As the owner of the house, I'm acting as a go-between for our builder, trying to get answers to this question. SO, forgive me if I don't give you all the info you need to get the answers WE need the first time around.

So, from what I understand, here's the problem. We don't have a top-plate. We're using a 2x6 ledger under the floor joists. Each floor joist sits on that ledger and is nailed directly to a stud. So the 12' on center doesn't work because the studs are 24 OC.

So, are we back to engineered joists as the only solution?



Blocking.  Nailing it to the stud does very little other than bracing and helping with a little rafter push.  The side nailing is not relied upon for load.  Simpson ties also work.  There are many ways to do it.  What you are trying to do is just keep the joist in place.  At 12" O/C the subfloor, a simpson tie and blocking should easily be able to do that.

Not exact, but a decent idea of blocking between joists.

John Raabe

Yes, blocking will take this up. See the detail on sheet 5 "Loft Floor Details". Note the fire blocking that is called out on top of the floor joists when they are nailed to the studs at 16" o/c. You can move that blocking down to the bottom of the joist resting on top of the ledger and between the joists. Then the studs and joists do not need to line up.
None of us are as smart as all of us.