Im working on some plans which will be checked by an engineer but would like to do a decent job to begin with and would like to educate myself more.

I have a loft with an LVL beam resting on a (4) 2x6 column and would like to figure out the size of the supporting joist to add under that. I've played around with calculation for section modulus, support reaction etc but missing the part on how to translate that to which size timber I would want to use and looking for resources/help on this.

As seen in diagram the loft has also a mid support beam so my column will support something like 59"x56" at lets say 40psf live load going into column yes? I think max deflection on IRC2018 for floor is L/360. My floor joists are 2x10 24"o.c. w center blocking with a clear span of 126".

How would I take let's say a 2 ply LVL 1.75x9.25 (specs) and see if that worked?

I'd love to learn the calculations within reason (within the scope of simple wood framing like this with a single point load). I've looked for videos etc but not finding the right thing - either too simple (continuous load) or too complicated and not covering how to apply to timber selection. Willing to buy a book/course if there is something good.

Thank you!

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  • 1
    This looks like a work assignment or a homework problem, not a DIY problem.
    – JACK
    Commented Apr 22, 2023 at 22:36
  • The support you'd add under that would be another beam, even if there were joists parallel to it.
    – Ecnerwal
    Commented Apr 23, 2023 at 2:51
  • 1
    Beam, joist.... yes - it will be a multi ply or fat LVL/PSL. @JACK - no its as much DIY as you can be - DIY engineering :). This is my house plans but while i have hired an engineer (actually 3... lets not go into that) the current one is shit, am out of time and this is the final piece. He claims I need a 5 -ply 2x10 which I find had to believe for this small section of loft- after he made a dozen mistakes I called him out on I dont trust it. Ultimately this is checked by my code examiner but want to do my homework. Commented Apr 23, 2023 at 4:32
  • Can you pour concrete at that point and put in a short column that will take the load directly to the ground?
    – Mark
    Commented Apr 23, 2023 at 15:12

1 Answer 1


You want to pickup the point load from a 4 member 2x6 post with a beam. Posts will almost never fail as they are just in compression. Beams will fail.

I feel like a 5 member 2x10 beam is the safe answer that doesn't require calculation. Something less than that then you really need to run the numbers and ensure the post load is calculated correctly. If you do a 5 member 2x10 that is maybe based on the load the post could support ( ignoring the actual load it is supporting ). Even if the 5 member is overkill how much cheaper is it to go to a 3 member beam ( not worth the time spent optimizing the beam size ).

If you use a program like forteweb (free to use) and put the design into it then it will tell you the sizing. I don't know that what you are asking is actually something that people will do as either the programs do it for them or the time required to ensure an optimized member will cost more than just putting in something that is stronger and you get more value out of a stronger beam than the time spent calculating to get a smaller beam accepted.

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    yes you are right. I think after seeing all the mistakes one engineer did I wanted to check myself - Ill look into Fortweb - I was hoping to find a nice Udemy course or YouTube on this so that I could learn the deeper workings but there is little in the way of bridging the math to actually figuring out the LVL size that I could find. Commented Apr 23, 2023 at 18:39
  • if you find something easy to digest I'd love to hear about it. I bought a structural engineering book but it was mostly math and not great as a self guided dive into it. I haven't played with forte but my lumber yard engineer used it but be interested to hear how you make out putting your plans into it. Commented Apr 24, 2023 at 21:20
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    Ya I am using Forte which gives me what I need - was interested in the math though. I found some ok vids on YouTube but there is little in the way of bridging the calcs to the wood specs though I am figuring it out little by little. Commented Apr 26, 2023 at 17:21
  • If you figure out a good way you can put your own answer to this question so others can search and find it in the future. I'd be interested for one. Commented Apr 26, 2023 at 19:10

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