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This is for a deck. If I make a beam out of two 2x10s, what would the maximum span be? The beams will be 8 feet apart (joist span). They will be resting on posts. The number of posts depends on how far the beams can span. This will be standard pressure treated lumber. I was going to use screws to hold the beams together. Maybe construction adhesive, too.

I have a book that says 8 feet, but want to double check that I'm reading the tables right. this is under a deck, no crazy loads like a hot tub.

Thanks!

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    Beam for what? That seems important. Construction adhesive isn't a structural component. – isherwood Jul 17 at 18:36
  • I presume that when you say "stacked" you mean the broad faces, not on edge... Also, details like how far they'll be spanning, will they be resting on posts or other beams, etc. – FreeMan Jul 17 at 22:04
  • Sorry, for a deck only about 3 feet above the ground. – BrianK Jul 18 at 1:28
  • Do you know the species and grade of wood? – Lee Sam Jul 18 at 1:37
  • construction adhesive will help to keep moisture out of the seam, that's a good thing. but don't count on it adding any strength. – Jasen Jul 18 at 1:48
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The Code requires a Live Load of 40 lbs. per square foot (psf) plus a Dead Load of 10 psf for a total load of 50 psf.

You indicate the beams are 8’ on center, so the total load on each beam is: 50 psf x 8’ (4’ on ea.side of beam) = 400 plf

In addition, the code requires an adjustment for wood being pressure treated. If wood is pressure treated without incising then it’s a 5% reduction. If it is incised, then it’s a 20% reduction.

Summary: Depending on the species and grade, 2-2x10’s can span 12’ with pressure treated lumber without incising and 10’-6” with incising. (I’m using a species of SPF and a Grade of No.1)

Note: If the beam is along the edge of a deck and is loaded with a single span, then 2-2x10’s can span about 15’ without incising and 11’ with incising.

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In general a double 2x beam can support a span in feet equal to its nominal width in inches.

A double 2x10 can thus span 10 feet.

Double 2x12 spans 12 feet etc...

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  • Is this rule of thumb for roof loads with 30 psf or floor loads with 40 psf ? Is that for exterior walls where it’s loaded on one side or interior walls where it’s loaded both sides? – Lee Sam Jul 18 at 2:57
  • It’s for typical deck construction. That’s what op is asking about. – Kris Jul 18 at 2:58
  • Oops, sorry. Is it for deck beams loaded one side at perimeter or interior beams loaded both sides? – Lee Sam Jul 18 at 3:24
  • Ask a separate question with those specifics. Then wow us with your answer🆗 – Kris Jul 18 at 3:27
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Use the prescriptions from AWC.

A beam must be 3 members unless you are getting a PE to do the design for you or you are using southern pine. The beam span also depends what your joist span is between the beams/ledger but lets assume 6' then with southern pine it would be 10' 1".

https://www.awc.org/pdf/codes-standards/publications/dca/AWC-DCA62015-DeckGuide-1804.pdf

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    Why does the beam need to be made up of 3 members? The diagrams in your attachment on pages 1, 3, 4, etc. show a beam made of 2 members. (Btw, those charts are for “single” span joists resting on a beam.) – Lee Sam Jul 18 at 9:48
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    The op says the beams are 8’ apart, not 6’ – Lee Sam Jul 18 at 10:00
  • @LeeSam I didn't notice the "or" portion of Table 3A that shows the 2 member beams. Not sure why they show the 3x material first. Thanks for pointing out the mistake. I'll edit the answer. Note 4 on table 3A says joist hangers - the charts are good for either hanging joists or for resting. – Fresh Codemonger Jul 20 at 17:12

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