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Zip vs. OSB Sheathing for Roof Deck

beedigs | Posted in Energy Efficiency and Durability on

Roof deck: Grace Ice & Water Shield + Regular OSB OR Regular Zip 7/16 sheathing? CZ2

We have decided to do ZipR3 on the wall sheathing for our one-story all-brick house.  Upon reading the Huber Zip installation manual, it says:

“When original roofing or claddings are removed and replaced on existing ZIP System sheathing, the roof or wall should be covered with an additional roofing underlayment or weather resistive barrier prior to installation of the new finished roofing or cladding.”

This made me think that perhaps using the regular Zip sheathing for the roof deck might be an overkill and better off using that money on other airsealing details for the house, since for asphalt reinstallation down the road we still need an additional roofing underlayment or WRB.

Would using just regular OSB for the roof deck and just taping the seams with Zip tape and then adding Grace Ice & Water shield before putting the asphalt shingles on be ok in our scenario?  Or should we bite the bullet and complete the house with a whole Zip system setup?  I am confused on what type of vapor permeability we need for our conditioned attic that will have a 5.5″ spray foam cavity insulation on the underside of the roof.

Please help lead us in the right direction.  We just want a good setup that can handle all the humidity and rain we get in Houston TX area but also want to make sure we prevent any condensation issues that would lead to mold which is what we are trying to avoid.

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Replies

  1. Patrick OSullivan | | #1

    Here are the advantages to Zip on the roof:

    1. It is useful when your roof sheathing will be exposed for an extended period of time.
    2. It is (at least in my market) cheaper and more readily available than 5/8" Advantech, which is another sheathing I would consider for extended exposure.

    My house has 5/8" Zip on the roof. It was great for the long period before I put an actual roof on. However, I put underlayment over it anyway because it's cheap, roofers are used to doing it, and it's just another layer of insurance in the event of a roof disaster.

    If I was going straight from sheathing to roof install, I would have used 5/8" CDX with the seams taped.

    1. Brian Wiley | | #2

      Hi Patrick, would you mind sharing what type of underlayment you used over the ZIP?

    2. Aaron Hawkins | | #4

      This is exactly what I would recommend if you're installing solo, or with a delay. I had both in my scenario.

      I installed Zip because it could deal with the exposure while I was waiting on my standing seam roofing to be installed.

      @Brian Wiley - I used FT Synthetics underlayments.

      https://www.ftsyn.com/ftplatinum/ - This got used on the majority of the roof deck.

      https://www.ftsyn.com/platinum-htsa-underlay/ - This was used on all the edges and valley.

      Both of these were chosen for their high-temperature compatibility since I was going with a black, standing seam roof. I really liked working with both products, they're very easy to walk on, and forgiving for a solo install.

      I would just make sure to match roofing material with underlayment.

      1. Brian Wiley | | #5

        Thanks for that info, Aaron. I'll be in a similar situation—dark standing seam roof, solo install—and those look like solid products.

    3. beedigs | | #9

      Patrick, thanks for your comments. Would the typical felt underlayment under a Zip sheathing suffice, or is it practical to upgrade to the Grace Ice & Water Shield (CZ2 - lots of rain in Houston)?

      What about vapor permeability (which is another big topic that my inexperienced mind has yet to fully understand) for the Grace product? I was led to believe that with our climate being zone 2, a layer of closed cell foam right under the roof deck followed by open cell spray foam fill to the rafters is not a good idea since that would lead to vapor not being to diffuse out through the roof? So, if the Grace product has zero permeability, would that be a wrong move? Or would felt underlayment be enough for our needs?

      Typically the builders here use regular 7/16" OSB for the roof deck ($20/sheet). 5/8 CDX is $50/sheet. Regular Zip 7/16" is around $50/sheet as well.

      What combo should I use? Zip 7/16" with typical felt underlayment? Regular OSB, seams taped with Zip tape, combined with Grace Ice & Water shield? 5/8" CDX, seams taped with Zip tape, combined with felt underlayment? We will be having our framing meeting tomorrow so will need to figure this out and make our decision asap. I appreciate your help and feedback.

  2. Expert Member
    Malcolm Taylor | | #3

    Beedigs,

    We cover the roof immediately after sheathing, so you are weatherproof and what material is used doesn't matter. Most underlayment are rated for a three to six month exposure.

    1. beedigs | | #7

      so i can with regular 7/16 osb combined with grace ice & water shield and that would suffice? although since we are trying to go with a belts-and-suspenders approach on water management wherever we can afford to pay more, sounds like Patrick's (comment #1) to use CDX plywood is good too in case there would be a leak. Now to check the prices (gulp!)

  3. John Clark | | #6

    When a roof is stripped the decking gets beat up and is full of holes from fasteners.

    Huber doesn't want homeowners to think that the WRB remains intact so they recommend a typical roofing underlayment.

    1. beedigs | | #10

      so perhaps Zip 7/16" for roof deck with typical felt underlayment for now, then later for roof reinstallation down the road we use Grace Ice & Water Shield underlayment if the Zip roof deck is still good minus the fastener holes from before?

  4. beedigs | | #8

    Makes sense..thank you

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