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Why do exterior decks not use 2x12 or 2x10 lumber?

Home Improvement Asked by wallyk on November 1, 2021

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 2×6 planks.

A few years ago I constructed raised garden boxes inspired by wonderful pricing at a lumberyard sale on 2×10 cedar; they have a similar sale every Spring.

Is there a good reason not to use 2x10s or 2×12 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.

One Answer

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.

Answered by Lee Sam on November 1, 2021

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