Summary: Looking for advice on allowed cantilever length for a multiply-supported joist.

As described in Deck Magazine (which cites the American Wood Council's construction guide) there are limits to how far a deck joist (also beams) can cantilever.

The typical / simple case is shown in the first diagram. The cantilever can't exceed 1/4th of the joist span:

enter image description here

I'm looking for advice on interpreting this rule in the situation where the joist runs of a series of intermediate supports. I've hacked up their diagram to illustrate this:

enter image description here

So in this drawing the the joist is continuous for distance L which could be potentially much longer than what would be feasible in the first diagram. And though this diagram only shows one intermediate support 'B' but you could imagine there being several.

But is the 'L' in the second diagram accurate? (Allowed?)

My assumption here is that one reason for the L/4 rule is to prevent a 'seesaw' effect. So using intermediate supports to increase L would still prevent this. But there could be other considerations I'm not thinking of...

(Note - I'm aware that the local code official will have their own point of view.)

  • Holding the main part DOWN is as important as holding it up, when all your friends come over to your birthday party and all stand on the cantilevered portion. (Bigger the cantilever, bigger the uplift.)
    – Lee Sam
    Commented Mar 3, 2019 at 17:38

1 Answer 1


The cantilever can't be too long for two reasons. One you want it to be stiff, so you don't have a diving board effect on the cantilevered part. Two, it can't be so long that weight on the cantilever part has a see-saw effect and lifts the other part of the deck off its supports.

I think you could certainly make the argument that you could cantilever (AZ/4) as long as the joist is sized to span AZ. (According to the cantilever joist span tables.) If that is the case, the supporting beam at B is not required and I see no reason to shorten the cantilever because you have an extra support.

Otherwise I think you'd need to have an engineer's stamp on the plans to use something between AB and AZ, up to one-fourth of the maximum span of the joist per the table.

  • I think you could argue (without an engineer) that a cantilever of the smaller of AZ/4 and Lmax/4 would be OK (where Lmax is the maximum allowed span for a joist of that size). But an engineer's say so would be even better. Commented Mar 3, 2019 at 15:17
  • The joist member sizing is an excellent point. I guess in my question I was assuming proper sizing would take place once the member lengths were determined, but you may be right that the L/4 guideline doesn't in practice allow you very much flexibility there. Mid-span supports could be used for stiffness but that would seem to be over-and-above what would be necessary for strength. Commented Mar 3, 2019 at 18:44

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