I'm working on a 7' long hanging bench with no support under the middle. I have 9' by ~6" rough sawn 4/4 white oak.

My initial design idea involved making a lamination / beam consisting of 3 boards ripped to 4" and cut to 7', making the beam dimensions 7' by 4" by roughly 2.625" thick.

However, despite the fact that I currently have 9' boards, I decided to go down this rabbit hole of looking into whether it would be feasible to cut the boards to a shorter length (e.g., 3.5' each, half of the needed stretcher beam length), then glue them up / join them to make the 7' stretcher from them.

This may seem ridiculously arbitrary, but I have 2 practical reasons for exploring this:

  1. shorter boards are more available (incl scrap between projects)
  2. shorter boards are much easier to S4S

I started looking into scarf joints in timber frames, and I also had the idea to glue the 7/8" thick boards all up in an overlapping fashion, so the beam would not contain any through-joints, only overlapping boards of varying length.

Am I barking up the wrong tree or is this something commonly done? Is there a way of achieving the same dimensions without losing double digit percentage of strength / load bearing capacity?

Note: I plan to use only wood and glue, no metal (e.g., brackets, etc.)

  • Are you asking about the title or actually something more? If you want to ask about just the title, trim the body of the Question until if addresses that, and the ultimate goal/need without various tangents. As for the Q in the title, I would suggest doing a couple of tests with offcuts glueing their ends together and see what the result is.... I'm presuming you'll be shocked at just how weak this joint is if you're even considering it.
    – Graphus
    Feb 26 '21 at 7:50

It is never a good idea to join shorter boards to create a span to carry any significant loads.

Having said that, there will always be situations where the beam may be longer than available material and joining pieces will be a necessary option. The actual layout of the 'beam' pieces is critical. You have not shown the actual arrangement of the boards you wish to join, so I cannot comment directly, but you should consider an important structural factor called 'bending moment'. In a simple span (beam supported at both ends), the point of maximum bending force (moment) is at the center of the span. In addition, the weakest point of your laminated beam will be where two boards butt to each other. If that joint is in the middle then you are aligning the weakest point of the beam with the location of greatest stress. So, for instance if you laminate three pieces of wood to create your beam, I would place the center joint at the center and the two outside joints as far from the center as possible. The closer those joints are to the end bearing points, the less bending moment (and more strength) they will have. It is also important to make certain that the beam pieces fully adhere to each other with a good glue bond and/or mechanical fasteners.

In any event, this handmade composite beam will not be nearly as strong as full span materials. I would never recommend it for any structural application without consulting a structural engineer

  • Thanks a ton for your answer. I was afraid this might be the case. I will stick to full length stock for sure then.
    – orokusaki
    Feb 28 '21 at 14:50

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