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How thick should my roof sheathing be?

user_6992381 | Posted in General Questions on

Can someone direct me to blogs or Q&A titles that address roof sheathing thickness?

My cabin trusses will be 24″ oc and I will use either 1/2″ or 5/8″ osb sheathing and a standing seam metal roof. The cabin is just 440 sf and is in zone 6. Roof will be fully vented. Is 1/2″ osb okay for a standing seam roof ( and other roofing materials ) or is 5/8″ preferred?

Tony

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Replies

  1. CMObuilds | | #1

    I am zone 6/7. Everyone including myself in my metro and sorrounding will typically use 1/2", I have come across 3/8" plywood on homes from the 80's. Typically if Im in northern WI zone 7 5/8 is the norm regardless of roofing type for the heavier snow load.

    I use 1/2 under standing seam with no problems, we get maybe 60" of snow a year average.

  2. Expert Member
    Michael Maines | | #2

    Tony, builders sometimes denigrate the building code, and it's not perfect, but a lot of smart people have spent a lot of time developing prescriptive recommendations for almost all of the typical construction situations. Table R502.3.2.1.1(1) in the IRC provides guidance on roof sheathing: https://codes.iccsafe.org/public/document/IRC2015/chapter-5-floors, and manufacturers' literature has more information. A lot of it depends on the live load where you are, which includes wind and snow loads. If you use edge clips (or tongue-and-groove material, though I've never seen that on a roof) you can use a thinner sheathing. In my zone 6 location with a 50 psf snow load, I use 5/8" sheathing on both 16" o.c. and 24" o.c. roofs. It is more than required by code but provides a nice stiff surface.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    T and Michael,

    Any thoughts on OSB vs plywood under metal roofs? Any concerns about holding power of screws into OSB? I've always used either 1/2" or 5/8" plywood.

  4. Expert Member
    Michael Maines | | #4

    Malcolm, I almost always use 5/8" ZIP sheathing on roofs, and metal roofing installers have never questioned it. I've never used commodity OSB on a roof but it's probably fine in 5/8" thickness, and possibly in 1/2" thickness. CDX works fine in 1/2" or 5/8" thickness. Some roofers or manufacturers don't like to install over skip sheathing, but I've never had a problem with it locally. I would strongly recommend talking to your roofer about their requirements before settling on your other roof details.

    The bigger issue, in my opinion, is making sure the appropriate roofing underlayments are used. It sucks to watch Ice + Water Shield melt down a fascia. (In the "Lessons that only need to be learned once" category.)

  5. user_6992381 | | #5

    Michael,
    Speaking of underlayments, what do you, and others, recommend between OSB and standing seam metal in zone 6? Small cabin, 4/12 pitch, vented.
    Tony

  6. Expert Member
    Michael Maines | | #6

    Tony, my favorites are Siga Majvest or Pro Clima Mento 1000, but there are many to choose from. Titanium UDL and Rooftopguard are two others I often spec. You also need a high-temp membrane at the eaves, at least; Protectowrap Jiffy HT and Grace Ultra HT are the two I see most often.

  7. user_6992381 | | #7

    Michael and all other responding advisers,

    Thank you for your responses.
    I've always been one who likes to know the WHY behind the answers to questions but I know it requires more time on your part. It greatly increases my understanding of whole systems.

    I have inferred from your answers that metal roofing is hot and will melt regular ice & water shield. Am I correct and is this true even in northern Michigan?

    And when you say "...at the eaves, at least", are you referring to code requirements for ice dam protection? "At least" seems also to imply that the ice & water membrane should ideally cover the entire roof (which I fully agree with even tho it's more expensive).

    I have also inferred (still without a why) that, unlike asphalt shingles, metal roofing needs to be isolated from the ice & water membrane which I assume (but do not know) is due to different expansion/contraction rates and also to prevent the metal from sticking to the membrane.

    Thank you very much for your time and patience. Your advice and explanations are invaluable to me and to many other readers!

    Tony

  8. Expert Member
    Michael Maines | | #8

    Tony, same here, I need to know "why."

    Yes, metal roofing gets very hot in all climate zones, and can melt regular I+WS. Code requires it at the eaves (projecting 24" from the exterior wall in plan view) but I also like to use it at transitions from roofs to cheek walls, around chimneys, and in narrower strips along rakes. But I've always worked in windy environments; it may not be as necessary where you are.

    I don't know about the separation issue. I don't see why you would need a separate isolation membrane between the roofing and the membrane, but there may be a reason. I would ask your roofing rep, and/or a local design professional, for input, and let us know what you find out. My typical detail is I+WS directly against the sheathing and over the top edge of the fascia, and a full coverage of underlayment membrane (with dripedge between the two for asphalt or wood roofing) but there are various ways to do it.

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