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It is time to replace the exterior decking (mostly cedar, but some planks replaced by Douglas-fir). Alas, the finishing work 12 years ago did not endure; some planks have rotted— particularly at ends and where fasteners penetrate. The deck uses 2x6 planks.

A few years ago I constructed raised garden boxes inspired by wonderful pricing at a lumberyard sale on 2x10 cedar; they have a similar sale every Spring.

Is there a good reason not to use 2x10s or 2x12 for decking?   Fewer planks.   Fewer fasteners per unit surface area.   More robust boards.   Seems like many advantages, but the lack of examples of actual use seems foreboding.

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There’s many reasons: 1) twisting, 2) cupping, 3) aesthetics.

  1. A larger board will twist more than a smaller board will as it dries out. As it dries out, the grain will shrink and a larger board will have a larger variety of different sized grain throughout (across) the board. This will cause the size of the gap between boards to shrink and enlarge as it dries out. This will be unsightly.

  2. As the boards are installed, the boards will need to be inspected (looking at the end grain at the end of the boards) so the grain is installed with the curve downward. With a large board, the grain at the end of the board will be more of a “S” rather than a “C” (that can be turned downward). This will allow part of the board to cup and collect and hold moisture, which will cause rot, etc.

  3. Seeing a large board laying flat is a bit unusual. Yes, because a smaller board is more customary it’s more aesthetically pleasing.

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  • Thank you for the thoughtful answer. By extension of your logic, maybe decks should be made of 2x2s?
    – wallyk
    Jul 21, 2020 at 1:26
  • @wallyk Clever...but you need some width to keep the boards straight.
    – Lee Sam
    Jul 21, 2020 at 1:59
  • As an additional point (that I've recently discovered on my own deck) - the slightest bit of cupping, especially on a freshly sealed deck, will lead to standing water in the cupped areas. Not only is this bad for the wood, it's inconvenient for walking (you have to dodge puddles) and it makes your shoes wet! (No, my deck does not quite have enough slope to allow the water to completely run off. :(
    – FreeMan
    Jul 21, 2020 at 12:36
  • @wallyk 2x2 lumber also has a nasty tendency of splitting when screwed down, even if it's predrilled.
    – FreeMan
    Jul 21, 2020 at 12:38

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