Does anyone have experience with pushing the limits of joist span for composite decking? My joists are generally 16" OC but a few gaps are up to 17.5" max (one has a short section at 18"). I'm worried that the composite or even 5/4" pressure treated pine will produce a bounce. Everywhere I see 1" composite they all say max 16" for residential, 12" commercial (this is a residential project).

Is an extra inch or two going to make a big difference? Is there any easy(ish) way of fixing this short of moving my joists or installing intermediates? The easiest solution I can think of is pulling off all the joists and inserting a new one. This isn't especially fun though because they aren't spanning a parallel frame (they will have to be cut slightly to shift down).

  • 1
    If just only you with no inspectors and no guess/family to snub their toes, I would say go for it. Any inspector and/or guess with snub toes will make your life full of do-do with nasty lawyers. Manufacturers instructions for their products are almost law onto themselves.
    – crip659
    Aug 7, 2021 at 21:41
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    So make the offending joists wider…
    – Solar Mike
    Aug 7, 2021 at 23:05

2 Answers 2


Sister another joist and add some more blocking. Second joist will cut down the width to 16" or just over, and blocking will help prevent additional flex that Trex has.


We just used Fiberon composite decking to build deck stairs and the deck itself. As my first stairs project, we only put 3 stringers with about 20 inches on center.

The deck itself had 16 in OC span of joists. The difference is noticeable but not significant (IMHO). It seems that 17in OC is negligible difference especially since you will fasten rather long spans of the decking.

Adding blocking (screwing board into the joist) is definitely a solution. If you have access under the deck you can do it after the installation, if you find bounce problematic.

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