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Thread: Roof Deck - what thickness and grade of plywood?

  1. #1
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    Nov 2007
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    Roof Deck - what thickness and grade of plywood?

    I apologize if this is not the correct forum for this question.
    What thickness and grade of plywood should be used for my roof decking?
    I own a bungalow that was built in 1960. My 25 year shingles have just about fried off (in 14 years) and every time I go up there to check things out the roofing plywood is so weak I think I am going to end up in the living room. The boards are not rotten, they are just too thin or too weak or both. I have decided to replace the roof, plywood, shingles and all. Is there any advantage to using plywood over OSB? Will I get better cost vs performance with OSB?
    thanks,
    Andrew

  2. #2
    Join Date
    Jun 2007
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    Quote Originally Posted by Andrew Thuswaldner View Post
    ...What thickness and grade of plywood should be used for my roof decking?...
    What does your local building codes call for? When my shop was built, it was decked with 1/2" CDX ply, but the requirement may vary with the local code authority and may depend on pitch and rafter spacing. I don't have enough experiece with OSB have a valid opinion on that subject.
    Tom Veatch
    Wichita, KS
    USA

  3. #3
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    Depending on your rafter spacing most roof sheathing these days is 7/16ths osb.

  4. #4
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    It's quite possible your current decking is 3/8" plywood. That was fairly common in the post-WWII construction boom, particularly houses built by National Homes. As for the plywood vs. OSB debate, oh boy, here we go again. I'll tell you that I don't think that of the hundreds of jobsites I've been on around here, I've seen anything other than 7/16" OSB used for roof decking.
    Jason

    "Don't get stuck on stupid." --Lt. Gen. Russel Honore


  5. #5
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    Oct 2006
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    Mid Michigan
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    I like 5/8" plywood or OSB but I am not the guy that has to carry the heavy stuff around. The guy that did my roof used 5/8" OSB after removing 7/16" OSB that had a lot of moisture damage from no attic venting. The 7/16" that was removed was swollen to nearly 3/4" in some areas. The OSB will last as long as it is installed correctly.
    The cover over my front porch was 3/8" plywood. It was very wavy and moved a lot when walked on. It is now solid as a rock.
    The guy that did my roof installed a lot of ice shield around all edges. Two runs around the bottom edges, a single run on the sides and a double on the peaks.
    David B

  6. #6
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    West Chester, Pa
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    I would use OSB if I were doing it. OSB that is made now holds up better than the old stuff used to. Your local code may require you to use 5/8" or some other thickness. If there is no such requirement 7/16" - 1/2" is fine if your rafter centers are 16". I would use 1/2" with clips in between the rafters if your centers are 24". You will have a little less waste with OSB since you can lay it in any direction where as with plywood you have to lay it perpendicular to the rafters.

  7. #7
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    1/2" CDX plywood is on my newly redone roof. The roofer originally spec'ed OSB (not sure of the thickness but 7/16" is typical) but I didn't feel good about that so I spent a little bit more and got plywood. You'll find varying opinions (some quite strong) on the OSB vs. plywood debate for sheathing.

    Cost wise, I think for my ~2400 sq. ft. of roof, the CDX was $400 more over OSB. Not big enough to not consider plywood.
    Last edited by Chris Padilla; 06-23-2008 at 5:50 PM.
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  8. #8
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    Nov 2007
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    Ottawa, Ontario, Canada
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    Thanks everyone for the input!
    Andrew

  9. #9
    Andrew -
    If your existing roof deck is not rotted, just overlay it with 7/16 osb.
    Make sure you use proper fasteners and follow correct nailing schedule.
    You'll be fine and its cost effective.

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