I am replacing this 20' long 3.5" x 9" wood beam due to termites. See pic.

I'm also replacing the 3 existing 4x4 posts with (3) pressure treated 6x6 posts.

Question is, can I swap out the beam (3.5" x 9" for (2) 20' 6x6 pressure treated posts stacked on top of each other (which would end up being a 6"x12"x20' beam?

And if so, should I glue them, bolt them, or just stack them? Or, would (1) 6x6 beam suffice?enter image description here

Best way to approach this?

  • 1
    Do you want the two 6x6 beams stacked on top of each other for looks? Sounds like major overkill if you want it for structure.
    – JPhi1618
    Sep 27, 2021 at 15:26
  • 4
    Where are you going to get 20' PT 6x6 timbers that don't cost an entire arm and part of a leg?
    – isherwood
    Sep 27, 2021 at 15:26
  • a 2x10 is going to be a ton cheaper and likely more rigid as well.And if you are resizing the height of the columns anyway, a 2x12 will definitely be more rigid. Sep 27, 2021 at 15:56
  • I purchased a 16' PT 6x6 about 3 years ago for around $60. That was prior to lumber prices headed into the stratosphere. 20's weren't available, so I don't know what the premium would have been for the extra 4 feet, but my 16' was notably more expensive per foot than a 12' was.
    – FreeMan
    Sep 27, 2021 at 16:09

1 Answer 1


The right beam is not a stack of posts. It's a doubled 2x10, which almost exactly matches your original beam's size. That ~10' span could also call for doubled 2x12s. You should ask your local inspection office what they'd require.

That said, you can dramatically oversize members with smaller heights to accomplish the same thing. Two 6x6 timbers would suffice. They'd also be much more expensive in most cases.

Just one 6x6 would almost certainly not meet code or load table requirements, resulting in sag and a safety concern.

  • Thanks. So, with the 6x6s, I just like the look of it, for the posts also. Would you suggest a glue-up, or any kind of joinery? Or, just stack em? Will also consult the local code, thank you.
    – brian l
    Sep 27, 2021 at 15:30
  • 1
    I don't have experience stacking lumber to create beams. In most cases, adhesives applied on-site aren't considered structural. You need adequate wood and adequate stability via steel hardware. I imagine a series of carriage bolts bottom to top would do. Again, consult your inspection office. Even if you're not getting an inspection done they'll tell you what would be required.
    – isherwood
    Sep 27, 2021 at 15:32
  • 2
    If just like the look for size, 3- 2x12s will give same size for less cost and be more structurally sound. Most structure members can be covered to look like what you want without going for extra costs for extra non needed size.
    – crip659
    Sep 27, 2021 at 15:45
  • if you really want to stack them then yes, glue lamination, is the way to do it. I don't know what type of glue wuld be right. most glue laminated beams are made in factories. you'll probably need an engineer to bless your manufactured beam.
    – Jasen
    Sep 28, 2021 at 4:51

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