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All the code books and specs I can find in terms of allowable cantilever distance is always shown from one edge only that is also the end that is furthest extended from the house.

Perhaps all the code books are specifying the cantilever distance allowable on any and all sides. This is what I would presume, especially with many decks not confirming to the typical rectangular shape, but I can't find anywhere that can confirm this.

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    Presumably because the cantilever applies to the joists and that's the way they usually run. What exactly is it that you want to build?
    – Olivier
    Jun 23 at 18:36
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    Semantically, no: "When we say cantilever, we are referring to any beam built into a wall that has a free end project." - "According to the new span tables and IRC provisions, cantilevers can extend up to one-fourth the backspan of the joist. This means that joists, such as southern pine 2x10s at 16 inches on-center, spanning 12 feet are allowed to cantilever up to an additional 3 feet"
    – Mazura
    Jun 23 at 23:16
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    You're talking about overhang. "Maximum allowable overhang cannot exceed 1/4 of the actual main span." (specifically though, a non cantilevered overhang, which might mean you have to use certain brackets to prevent upheaval, IDK.... +1.)
    – Mazura
    Jun 23 at 23:16
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    It's not hard to compute/model, it is simple lever mathematics. You were taught that in high school physics. A downforce on a cantilever pivots the board on the nearer support, causing an up-force on the farther support. How much up-force can it withstand? And what will your worst-case off-balance loading be? Jun 24 at 4:49
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    @Harper-ReinstateMonica while up-force should also be considered, I believe the primary consideration is the beam loading. Even if you were to attach the far end to an arbitrarily strong support, there are limits to what you can support with a cantilevered 2x. Jun 24 at 22:46

3 Answers 3

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The 2018 IRC (as adopted by Oregon, where I live) allows a free-standing deck to have both cantilevered beams and joists. The 2015 IRC (as adopted by Texas, listed in your profile) is similar but has slightly different diagrams.

Beams are described in R507.5 Deck Beams: "Beams shall be permitted to cantilever at each end up to one-fourth of the allowable beam span"

beam cantilever diagram

Joists are described in R507.6 Deck Joists: "The maximum joist cantilever shall be limited to one-fourth of the joist span or the maximum cantilever length specified in Table R507.6, whichever is less"

enter image description here

Note that a joist cantilever in this configuration requires the joists to sit on the beam ("dropped beam").

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    This is the answer I was going to post (but this has much better diagrams). It comes down to the definition of "cantilever", which I take to be an extension of the floor system beyond the foundation. In our case, the foundation is the posts.
    – isherwood
    Jun 24 at 18:28
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Decks are cantilevered on one side because of the directionality of the joists. With ledger support at the house, there is only one area that needs cantilevered, at the end of the deck supported by a beam.

In your hypothetical case, in order to need cantilevered on all 4 sides, the deck would have to have joists running in all 4 directions, with 4 beams in a square shape smaller than the deck size. To run in 4 directions you would need creative joinery - like an X shape with the rest of the joists joined to the X at 45 degrees.

Hypothetical square deck diagram

If you were building a square floating deck (not joined to the house as typically done) most would have just 2 beams perpendicular to the joist direction, which would have just 2 cantilevered sides.

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  • This is an excellent answer to my question. My question was a little ridiculous, as a couple comments noted that a cantilever off the side makes no logical sense (unless you build a deck the way you drew it). In my head, I made the "position of last girder/beam" equal with "position of support post," which those aren't the same always. Do you know if the support posts also allow the beam to cantilever off the sides or is cantilevering only for joists extending beyond beams, or can a cantilever also happen for the beam (meaning the support post isn't at the end of the beam/girder)? Jun 24 at 15:20
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    Ah, good clarification. Yes, as mentioned in comments on your question and in diagrams on another answer, beams can "cantilever" past the posts. I'm not aware of any Code-defined limits but they may exist, the diagram provided by Mazura matches up with my experience. It shows spans limited by beam dimensions and overhang maximum ~1/4 of the span between posts. Jun 24 at 16:06
  • Not exactly true. A combination of conventional and flush beams would do the job as well.
    – isherwood
    Jun 24 at 18:25
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I can't find any code for it, but my mind keeps saying you only get two feet of overhang on a floating deck... because the maximum spacing for posts supporting a 2x12 is 8', and one quarter of 8' is 2'.

"Maximum allowable overhang cannot exceed 1/4 of the actual main span."

And I guess w/e code this picture is citing, "the actual main span" is actually w/e the distance is to the next post because it's a freestanding deck.

enter image description here

https://www.pinterest.com/pin/520939881903050593/


Presumably when cantilevered into the building or attached to a ledger, you get to use the actual actual distance.

enter image description here

https://www.finehomebuilding.com/project-guides/decks/get-deck-joists-right

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  • This is a great answer! The pinterest picture indicates that a beam can also cantilever out from the posts. I mistakenly worded my question to equate cantilevering joists and beams (which is why books only talk about it going off the end, which I presume is fine for 2 opposing ends, but all 4 sides is ridiculous). I'm also trying to consider cantilevering the beam from the support posts off the sides like the bottom pinterest photo indicates. Do you know if the cantilevering for beams off the side like that is a) allowed, b) uses the same formula as the joist cantilever (or something else)? Jun 24 at 15:24
  • @TonyDiNitto - Check Evan's answer. It cites code and should be the accepted answer.
    – Mazura
    Jun 25 at 1:15

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