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This is an extension to a previous question. I would like to know what effect the type of decking used will have on the answers provided. This question comes from the fact that although the best option from the previous question seemed to be moving up to 2x12, I don't think I have the clearance for it. Therefore, I have to stay with 2x10 framing and need to consider the other ways to provide extra stiffness.

I am leaning toward using a composite decking. For the sake of the question, let's work with the assumption that we're using the typical sort of composite, like a Trex or a TimberTech just to name two. Those seem to come in 1" (0.94") thicknesses, at least as a standard.

Ultimately the question just boils down to whether I gain or lose stiffness just by choosing composite deck boards. Then I can consider what else I have to do in the framing beyond that in regard to the overall stiffness problem.

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The decking is not the main structural part of a deck. That should be the posts and the joists. If you need more support or stiffness, add larger joists. If you have no clearance for that, add MORE joists at closer spacing.

While the deck itself does add some stiffness and support, that's secondary and it will not do what you are wanting.

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  • That was all addressed in the linked question. Since, as you said, there is some effect to be gained, this doesn't answer the question.
    – isherwood
    Commented Mar 8, 2021 at 15:46
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It's difficult question to answer because there are many variables. Among them:

  • Composite decking material
  • Wood type
  • Wood treatment
  • Wood age
  • Climate and sun exposure

The particular brand of composite decking can dramatically affect stiffness. The early recycled plastic decking was as floppy as a noodle, especially when warmed in the sun. Later products became so hard that you had no choice but to countersink for every screw or they'd strip out in the joist.

As I mentioned, sun exposure is a factor. The difference in stiffness at 50 degrees (morning shade) and say 100 degrees (afternoon sun) is profound. Which are we considering here?

Then, what's the control decking? New pressure-treated SPF decking is quite flexible, but heavy. Those traits counter each other somewhat in terms of walking feel, but if we focus on flex, new PT decking may actually be less stiff than composites. That all changes as it dries and ages, though, and again when it begins to degrade after a decade or so. Cedar tends to start soft and flexible and stiffen only slightly with age. Redwood is so rare and expensive that I'll disregard it. What about teak and other "exotic" wood? They may be harder and stiffer.

Here's a rough ranking, in order of increasing stiffness. It's about the best I can offer.

  1. Aged, dried 5/4 exotic (harder) wood
  2. Aged, dried 5/4 PT SPF
  3. Harder composites in the cool shade
  4. Aged, dried cedar
  5. Softer composites in the cool shade
  6. Harder composites in the warm sun
  7. New cedar
  8. New PT SPF
  9. Softer composites in the warm sun
  10. Decaying wood
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  • It’s difficult to compare “stiffness” of two different deck designs, because of all the variables. However, one significant affect is that composite decks are fastened to supports with clips (designed to let the board move) while “boards” must be fastened (nailed) directly to the support. Horizontal stiffness can be increased using brackets to fasten to the house (which are now required by Code), while vertical stiffness is deflection. Deflection of decking, joists, beams, etc. is based on species, grade, and spacing. I could make any system stiffer than another just by changing the spacing.
    – Lee Sam
    Commented Mar 8, 2021 at 16:49
  • That's a good point. When I wrapped up my building career most composite decking was still screwed down. Hidden fastening systems change things somewhat, though maybe not so much with respect to vertical movement.
    – isherwood
    Commented Mar 8, 2021 at 17:00

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