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If the joists are 16"oc and our plywood is exactly 4x8' and we have to allow 1/8" gap between our plywood panels wouldn't it shift the next panel by 1/8" and then the next by 2/8" and eventually some panel won't have enough of a joist to lay on?

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Looking for "sanded" plywood and check the specification for the panel you are looking to purchase. This one is a 4x8x3/4 from Menards. https://www.menards.com/main/building-materials/panel-products/tile-backer-board/4-x-8-acx-sanded-plywood/1251420/p-1444425371729.htm

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  • So it is 1/8" shorter on the 4 and 8 foot sides, making the 1/8" spacing automatic.
    – Jon Custer
    Commented Sep 15, 2021 at 13:27
  • In other words, they thought of that. Most standard lumber (in the US at least) is slightly smaller than advertised (e.g. a 2x4 is actually 1.5"x3.5"), just to allow such gaps. (In the case of planks like the 2x4, it's to account for 1/2" of drywall) Commented Sep 15, 2021 at 13:29
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    There are loads of reasons, @DarrelHoffman, that a 2x4 isn't actually 2" by 4", but "allowing for the thickness of drywall" isn't one of them. For one thing, not everyone uses 1/2" drywall. Sometimes (wall between garage and living space) it's 2 layers of 5/8" drywall! You don't go buy 1-1/2" x 2-3/4" studs for that wall, you just live with the slight loss of interior square inches. And what explains the 1-1/2" vs 2" dimension?
    – FreeMan
    Commented Sep 15, 2021 at 14:18
  • @FreeMan I suspect the real reason is simply that this is "the land of the free". If you can advertise something as having 8 square inches of lumber, and get away with selling only 5 1/4 square inches, why would anybody want to do anything different? The only question is "why stop at 5 1/4 - why not 4, or even 3?"
    – alephzero
    Commented Oct 8, 2021 at 1:15

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