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I'm planning a replacement for my deck and ran into a question that I cannot find an answer to anywhere. Why does everyone place deck joists at either 16" or 24" OC (or sometimes 12"), but not 18" or 20" or 16.75" centers?

In my case, I have a setup where 24" centers would be within code, but I don't like the feeling of decking with joists that far apart. 16" centers are what I'm after, more or less, but that would leave me with a final space of about 4". Why not just space out the rest of the joists to 16 1/2" spacing and save a joist? I could leave a 20" space at the end, but again, why not space all of the boards evenly?

I can see why you wouldn't want to do this in an enclosed floor or wall inside a house, but for something like a deck where the framing is actually visible between the decking boards, why does it matter? Or does it matter?

To state the question(s) clearly: Are atypical, but regular, joist spacings within code? If so, why are they not used? If not, why not?

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  • I don’t like it, because it looks like a drunken Martian who didn’t know what he was doing installed them.
    – Lee Sam
    Commented Nov 17, 2017 at 3:18

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Because that works out with other common deck and building components and aligns with decades-old convention. For example:

  • Decking lengths are usually in 2' increments
  • Decking is built for specific spans with respect to strength and stiffness
  • Floor sheathing is 8' long (which sets the standard)
  • Joist bridging is sized for 16" centers
  • Insulation is sized for 16" centers (when you convert your deck to a three-season porch)

Etc., etc. Incidentally, 19.2" is also a common joist interval because it works out to 8'.

In your case there's probably no immediate reason you couldn't do something non-standard, if your decking is rated for the span you go with. Code probably doesn't address joist interval as long as it meets the requirements of the other system components.

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  • Are all the boards full length? Would hate to see some right on the edge.+
    – Ed Beal
    Commented Oct 9, 2017 at 18:06
  • I double if 1/2 will hurt anything, but keep in mind that when you go to put the decking on, a board might not reach if its comes in an exact foot measurement like 8,12,16, etc. You may have to add nailers so boards will reach, or cut them one joists short which will create a lot of scraps. Hopefully, boards are an inch or so extra in length.
    – BrianK
    Commented Oct 9, 2017 at 20:50
  • @BrianK Decks are often built in sizes other than 2' increments. For any deck under 16', the length of available decking boards is completely irrelevant to the spacing of joists in any case, as that spacing is interior, in the middle of the deck boards.
    – Jason Patterson
    Commented Oct 9, 2017 at 23:21
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In my school of thought, which may be different from someone else's, if my deck joists are going to be 2X8 or larger, I use 16" centers. If they are going to be 2X6 joists, 12" centers are what I go with. I have never, nor will I ever, build a deck with 24" centered joists. That is just way too far apart. You want your decking to have as much support under it as possible so you don't fall through. However, I really don't see much of an issue with going 16 1/2" on center, but personally, if it were me, I would go ahead and buy the extra joist to stay on 16". A deck is not something to cut corners or go cheap on. Too dangerous.

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