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How much of a gap should I allow in exterior plywood sheathing to allow for expansion and contraction?

user-6698999 | Posted in Green Building Techniques on

Thank you to everyone who has posted and responded to related questions and gotten me to the point where I can more or less ask this question in an intelligible manner.

I am remodeling a 1920s colonial revival one-story, conventional foundation home in central Mississippi. Much of the diagonal sheathing is still intact and in good shape, but for the master addition that is replacing an old 1960s add on, and for the garage (new slab on grade), we will be using plywood sheathing and Tyvek CommercialWrap D house wrap. The walls will be insulated with NuWool cellulose, and the siding will be Hardie Board. The contractor is opposed to taping the seams on the plywood because of expansion and contraction.

After reading Martin’s article entitled Backyard Tape Test, I am considering Siga Wigluv, Pro Clima, and 3M tapes.

Assuming that plywood expands and contracts enough to make this an issue (and please correct me on that if it is an incorrect assumption), should I leave a gap between the sheets of plywood to account for expansion and contraction? If so, how big should the gap be? Also, any tips on explaining this to my contractor? He is a smart guy with lots of experience, and he cares about his work, but he does not like doing things differently than he normally does.

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  1. Expert Member

    The standard gap is 1/8". An easy way to keep it consistent is to space the sheets with a common framing nail.

    Probably the best way to explain it to your contractor is to tell him it's the recommendation of the APA and most building codes.

  2. user-6698999 | | #2

    Thank you for responding, Malcolm. So the tape doesn't have a problem dealing with contraction and expansion? Asked differently, the tape will maintain its grip even as there is expansion and contraction?

  3. Expert Member

    Sorry, I only answered half your question - and I don't have enough experience with tape to answer the other half. Hopefully others will chime in.

  4. GBA Editor
    Martin Holladay | | #4

    To the best of my knowledge, based on many reports from the field, any one of the three tapes you listed -- Siga Wigluv, Pro Clima tape, or 3M All Weather flashing tape -- will be able to handle the small amount of expansion and contraction seen at sheathing joints.

  5. user-6000591 | | #5

    Thanks Martin and Malcom. I am expecting to receive my 11 rolls of Pro Clima Tascon Vana tomorrow. I appreciate all the help.

  6. mackstann | | #6

    This reminds me of something I've wondered for a while... if the sheathing is exactly 4'x8' and the studs are spaced exactly 16" or 24" OC, then don't the 1/8" gaps accumulate after a few sheets and cause them to stop aligning with the center of the studs? Do carpenters have to shave off a bit from some sheets due to this?

  7. GBA Editor
    Martin Holladay | | #7

    Unfortunately, the APA video (below) doesn't answer the stud-spacing question.

  8. Expert Member
    Dana Dorsett | | #8

    It takes 36' (nine 4' panels, 8 gaps) of wall before the panel creep has moved a full inch, which is probably the absolute limit. If sheathing the wall starting with the first panel in the middle rather than at one end, it should be just possible to sheath a 36' length of wall without having to trim a panel to keep the end panels sufficiently over the studs for nailing.

  9. Expert Member

    Most sheathing panels now come 1/8" undersized.

    For full dimension sheet goods there a number of strategies. Some framers "creep" their layout to compensate. Some cut 3/8" off every few panels - and some slam them tight and hope for the best.

  10. GBA Editor
    Martin Holladay | | #10

    Thanks for the link to the LP specifications. I suspected that was true for some brands of OSB and plywood, but was unable to confirm my hunch in my own search. You found a document that confirms it -- so thanks.

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