2

I am trying to "optimize" materials purchase when building a deck. So I got 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. If the answer is yes above, I want to buy the ones from Home Depot. But why is there such a difference in price vs. Lowes' 4x8x16?

2x8x16:

http://www.homedepot.com/p/Construction-Select-2-in-x-8-in-x-16-ft-Construction-Select-Pressure-Treated-Lumber-549000102081600/206931774

4x8x16:

http://www.lowes.com/pd_209870-10926-209870_0__?storeNumber=2420&selectedLocalStoreBeanArray=[com.lowes.commerce.storelocator.beans.LocatorStoreBean%405cb15cb1]&productId=3184283

3

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.

  • 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 '16 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. – Harper - Reinstate Monica May 17 '16 at 1:32

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