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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.
    May 15, 2016 at 6:49
  • 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. 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. 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
    May 25, 2023 at 16:03
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    this needs a reference May 25, 2023 at 16:40

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