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I know you should lap joists at least 12", and the longer the lap the better. However, in order to use 8' joists, can I lap the joists like the image attached?

The beam in red is a drop beam. The total lap would be 3'-1" long, but the 8' joist would only overhand the beam by 7'-1/4", so the lap would not be "center" on the supporting beam below. Is this ok?

Edit: I should clarify, yes the joists would be connected together (sistered) thoroughly, likely using a dense nail pattern, multiple 1/4" through bolts, but most likely Simpson Strong-tie’s Framing Screws.

enter image description here

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  • Both joists are being supported by the beam. I have seen where both joist ends were resting On the beam. The sistering is not doing the work here, the beam is carrying the load.
    – Alaska Man
    Apr 11, 2021 at 18:29
  • Right - I got that much. But I've read that sistering improperly can push the other end "up". So I just want to make sure that as long as I have a good 3' that it doesn't matter if the beam is off center and wont cause other problems.
    – Jason Roozee
    Apr 11, 2021 at 18:44
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    To me, “sistering” means adding another joist and fastening to an existing joist. You appear to be “splicing” directly over a support (beam). Am I wrong?
    – Lee Sam
    Apr 11, 2021 at 20:09
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    Technically you are not sistering, see jacks comment. The beam will not allow the joists to move down so the other end can not move up. (unless you have other more significant structural problems)
    – Alaska Man
    Apr 11, 2021 at 20:19
  • @P2000, your answer is relevant and insightful and I'd have upvoted it. Consider undeleting.
    – isherwood
    Apr 12, 2021 at 20:52

2 Answers 2

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As long as both joist go over the beam your are fine. Remember if it bothers you you could always cut one of the joist so it was equidistant over the joist but its total unnecessary.

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  • An more detailed explanation as to why "As long as both joist go over the beam your are fine" would be a more useful answer. As written you "answer" is just reiterating what is said in the comments. The longer over hang is not bothering him, the lack of both overhangs being equal (the shorter one) is what concerns him.
    – Alaska Man
    Apr 11, 2021 at 20:50
  • diy.stackexchange.com/help/how-to-answer
    – Alaska Man
    Apr 11, 2021 at 20:57
  • You got it Alaska Man. That's precisely my concern.
    – Jason Roozee
    Apr 12, 2021 at 17:13
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    There's really no further explanation needed here. Obviously each joist needs to rest on the beam, and the rest is moot. Plus one.
    – isherwood
    Apr 12, 2021 at 18:31
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IMO, theoretically, a well-jointed splice is as good as the joist without a splice. However, the stresses in the wood and fasteners will complicate the issue due to offsetting the splice on the support beam, at which the internal stresses (shear and moment) of the joists are very high, and each joist will have different internal stresses that are difficult to evaluate and may cause problems in the long run. The graph below shows the clamping forces required to close the gaps due to deflection of the joists at the splice, and the shear and moment of a continuous joist over intermediate support (beam).

I suggest either keeping the splice center on the beam or shift the splice away from the beam. Otherwise, pay extra attention to your splicing details.

enter image description here

At this juncture, you shall ask yourself "do I need to splice the joists to make it continuous or not?". If it is unnecessary, you shall trim the joists to the length required for a simple bearing connection, thus eliminating the problem of overhanging that will raise higher over the support and pushing against the floor in contact. Since this (offset splice) is not a common problem addressed by most trade publications, therefore, it pays for consulting an engineer or experienced wood structure designer if you intend to proceed but having doubt.

Your detail will not threaten the integrity of your structure, it may not fail immediately. Rather, potentially it would affect the long-term performance of the structure by building a defect into it. A forum like this is a good place to gather ideas, but you have to make the right call for your own good. Good luck.

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  • Why do you say “internal stresses (shear and moment) are very high over the beam”? Actually, moment is zero.
    – Lee Sam
    Apr 12, 2021 at 20:09
  • Nonsense. There's no place for clamping forces or fastener stress or torque arms here. None of that is even remotely relevant as the joists can simply stand on their own with no interconnection whatsoever. They're not structurally related to one another, and their overlap is arbitrary.
    – isherwood
    Apr 12, 2021 at 20:43
  • Your moment diagram is incorrect too. It looks more like the way the joists would bend, than a moment diagram. The moment between bearing points is positive, so the moment should be above the neutral axis line...not below it.
    – Lee Sam
    Apr 12, 2021 at 20:47
  • @ishwewood If you know how to draw the deflection diagram of each joist independent of the splice, you will understand where are the clamping forces came from. No matter the location of the splice, there is always internal forces, that's why we need connectors to connect two separate elements to make it a full/whole. The location of the splice does matter because the internal forces change along the joist. For better, the splice shall be kept away from where the maximum stress occurs, at the support and the midspan.
    – r13
    Apr 12, 2021 at 21:42
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    @r13 The op did not say the joists are fastened together. In fact, the question relates to what happens to the cantilever end and if it would be a problem if not centered on the support.
    – Lee Sam
    Apr 12, 2021 at 22:39

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