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I am trying to "optimize" materials purchase when building a deck. So I have two questions:

  1. Is two 2x8x16 beams the same as one 4x8x16 beam for deck support? Do I need to bolt / nail them together or just sit them side by side on top of the posts?

  2. <question about pricing removed since it's off topic>

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If you pair the beams up, you should glue them with construction adhesive. Just nailing or screwing them together won't really distribute the load across both beams. Although; you're probably not planning on hanging or resting anything on just one beam and expecting the other beam to help with the load anyway.

A pair of 2x8 beams are going to be 3" wide (typically 1/4" is planed off each side of dimensioned lumber to give it the finished surface).

A 4x8 beam is going to be 3-1/2" wide. The extra half inch is going to add some strength to the 4x8 beam.

The 4x8 might be more prone to warping than a couple of 2x8's if it's a solid beam (but not if it's a laminated beam).

If this was for indoor use, you could sandwich a strip of plywood between the 2x8 beams and glue (not nail) all three pieces together to create a laminated beam that would be stronger and less prone to warping than the 4x8. But since you're building a deck, the sandwiched plywood would soak up water and be a problem.

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    This is a good answer. Look like I will not use two 2x8 then because I do plan to use the joist hangers on both sides. Plus, dealing with adhesive gonna take more time than pay additional $10 for one 4x8 ;-) Just thinking out loud
    – HP.
    Commented May 15, 2016 at 6:49
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    Nowadays they don't actually plane 1/4" off each side of dimensional lumber. They cut it much closer than that. That's one reason the thicker boards cost more, they require more timber to make.
    – Harper - Reinstate Monica
    Commented May 17, 2016 at 1:32
  • Sure. Tomato/Tamawto, though. The net result either way is that a "2 by" piece of lumber is 1-1/2" wide, so two "2 by" pieces of lumber together will be 3" wide, and a "4 by" piece of lumber will be 3-1/2" wide.
    – Craig Tullis
    Commented May 27, 2023 at 0:13
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2 2x8 12% stronger than 4x8 beam if those are true dimensions, but, 2 2x8 = 3x7.5, and 4x8 = 3.5x7.5 (22% bigger)....beam wins by 7%. (look out for warping)

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    – Community Bot
    Commented May 25, 2023 at 16:03
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    this needs a reference
    – aaaaa says reinstate Monica
    Commented May 25, 2023 at 16:40

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