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Thread: Lap Joint

  1. #1
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    Default Lap Joint

    I was thinking that this lap joint should split over the post, but now I am second guessing myself. Perhaps the lap cut should not have been so deep? Anyone got a good visual for the proper joint. I'm picturing a lap joint not as wide, but is it supposed to be 1/2 and 1/2, or more of a 1/3 / 2/3 for the split on top of the post?

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    Jim Robinson
    New Mexico, USA

  2. #2
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    Default Re: Lap Joint

    Hi Jim,

    The bottom edge of both beams should be supported on the post, so that joint connection (I called it a scarf joint) is improper. Not many people use that kind of joint anymore. Most people will use a butt joint and some kind of hardware. Much more straightforward. It's kind of late and I am having trouble remembering minimum bearing. 3/4" is what comes to mind, but I believe that is for a joist. Can't remember if a beam/girder is different. Sooo tired....

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  3. #3
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    Default Re: Lap Joint

    If that work is less than a year old, I would be worried about that beam on the left rotating/twisting itself out of whack any more than it has. If it is older, I would just note it as not quite right. I can't give you the correct specs, although I have the answer in a book somewhere, buried archives.
    Some shoulder pieces could be added to the top of the post, but I'd like to see more good old diagonal braces.

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  4. #4
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    Default Re: Lap Joint

    Jim

    The beam on the right would probably be fine. The one on the left is subject to splitting. Simpson StrongTie makes a beam bracket (HH6) that would provide the additional support needed.

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  5. #5
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    Default Re: Lap Joint

    That's the way I see it as well. I'm picturing a lap joint like the one I sketched below the photo on the PDF. That would have, in theory, 2.00" of bearing on the short side of the post and 3.50" on the longer side, but still well over 1.50" of bearing.

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    New Mexico, USA

  6. #6
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    Default Re: Lap Joint

    I see a few long term concerns here. The lap joint itself obviously should have been done differently. Both legs should bear on the post.
    From a long term ownership and stability perspective the install overall is going to be a problem.
    - The left beam is already twisted and no longer sitting flat, if it ever was anyway; does not appear to be attached in any way
    - The right beam clearly has knots and running splits right at the edge of the post. Under heavy load conditions those cracks could run the rest of the way through the beam to the top side. Too toenails should not be considered sufficient attachment.
    - The entire assembly should have been properly through bolted or have standard steel angle brackets installed. Get a bunch of people dancing on that deck and there isn't anything to keep that joint from moving apart or off the post.

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  7. #7

    Default Re: Lap Joint

    If that were on my house, I'd just secure some nailers onto both sides of the post.


  8. #8
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    Sep 2008
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    Default Re: Lap Joint

    You are correct in both of your thoughts.

    The beam should set on the post at least 1 1/2" and the notch is too deep for that configuration.


  9. #9
    Stephen McSpadden's Avatar
    Stephen McSpadden Guest

    Default Re: Lap Joint

    As a structural engineer I have to indicate that both sides may be problematic. Randy is most correct; the left side is more problematic because it will be more prone to splitting horizontally from the inside corner of the notch that was cut to create the lap. The other side is also problematic because of a force at the end of the beam called "horizontal shear". If too much of the wood has been notched out it will also cause horizontal splitting in the wood but for a different dynamic. But the final answer is "it depends"! If the actual loads are light relative to the beefy size of these beam everything may be OK. It really should be checked out by a structural engineer. If found to be problematic I would recommend building a haunch or corbel on each side to provide adequate bearing and support.


  10. #10
    Bruce Grant's Avatar
    Bruce Grant Guest

    Smile Re: Lap Joint

    Hey all. I have been inspecting log and timberframe homes for quite some time after many years of building experience. That joint is, as most pointed out, assuredly wrong. The minimum bearing should be 1 1/2 in. If I saw any kind of lap joint whether it had bearing or not I would suspect homeowner construction. Properly I would expect to see that joint with the left side bearing 1 1/2 inches on post and then cut on a 22 1/2 to 30 degree angle with the right side cut on the matching angle. That allows both sides full 1 1/2 in bearing. The left side should bear above right. No visible metal needed with continuous load on beam. To help prevent twisting in longer beams, at least up here, they may be tied with metal tie plate on top where it can not be seen.


  11. #11
    Michael Avis's Avatar
    Michael Avis Guest

    Default Re: Lap Joint

    I agree with the comments posted previously all of which condemn the joint for different reasons. I add another reason as grist for the conversation mill...

    The effective depth of both beams is reduced by the notch leaving the beams to rely on shear strength at the support. The structural engineer alluded to this.

    So if the beam is supposed to be w" x h" for the span and anticipated loads, both of these beams are less than that because of the notch. Maybe not a lot less because the cut is right at the bearing point but less anyway.

    As a longtime carpenter it seems like a lot of work to make this lap joint to yield a weaker beam and a problematic connection.

    If the notch had been made along the vertical axis I think it would have been better but still not as good as having each beam bear fully at full depth at 1 3/4" assuming this post is a nominal 4 x 4.


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