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Discussion Forum

Plywood vs. OSB

CCI | Posted in General Discussion on November 21, 2006 07:59am

Quick question about the use of OSB.  I am putting a pull down staircase in my attic.  The area will just be used for storage of tote boxes filled with out of season clothing and Christmas stuff – Nothing very heavy.  I have done these before and always used plywood for the floor.  I was in HD and saw some OSB that was about 1/3 the price of the plywood.  Is there any reason not to use this for a “subfloor”.  It will be fastened to the 2x joists 16″ OC.  The height to the peak of the roof is about 5-1/2ft so no one will be walking on this just sort of crawling around.

I don’t need much – maybe 6 sheets so the total price is not that big a deal but why spend 3 times as much if I don’t need to?

Thanks.

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  1. user-201496 | Nov 21, 2006 08:11pm | #1

    Just use 1/2" (15/32) CDX plywood. If the joist are 16" o.c. you'll be fine. I would stay away from the osb.

  2. BobC123 | Nov 21, 2006 08:47pm | #2

    The two are considered identical structurally.  I use it all the time and  you can expect the same performance.

     

    http://www.umass.edu/bmatwt/publications/articles/osb_vs_plywood.html

  3. DanH | Nov 21, 2006 08:59pm | #3

    The only real problem with OSB for this duty is that it's a lot heavier. Otherwise, it may sag 1/16" more but who cares?

    Consider any future insulating you may do. Eg, it might be wise to put down 2x4s edge-up going at right angles to the joists to provide room for additional insulation.

    Don't cover up any electrical boxes that face the attic -- make a cutout for them or screw down a small panel over them, with the box position noted on the panel. (But it's OK to cover the backs of ceiling fixture boxes, etc.)

    People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
    1. CCI | Nov 22, 2006 05:43am | #4

      Is the OSB really that much heavier?  I need to carry these sheets up to the third floor.  The OSB is 7/16" and the plywood is 15/32".  I figured the 1/32" difference would not be a problem structurally but if the OSB is that much heavier I may stick with the plywood.

      Is there any problem with the OSB being stepped or knelt on between the joists?

      1. FastEddie | Nov 22, 2006 05:46am | #5

        Step up to 5/8" ply and rip it to 24" wide pieces ... easier to carry.  7/16 osb will deflect a lot between joists. 

        "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

      2. DanH | Nov 22, 2006 06:08am | #6

        Never weighted it, but my take is that OSB is 2-3 times heaver than the equivalent size and thickness of plywood.  (There is a fair amount of weight variation between different brands/grades of OSB.)

        3/4" OSB is fine on 2-foot centers -- 7/16" would be a little springy.  I believe you said you'd have 16" centers, so probably 7/16" will do fine.

        Of course, we're not exactly talking fine homebuilding here -- just rough attic flooring.  So long as the stuff doesn't collapse under you it doesn't really matter how much it flexes.
        People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck

        1. wrudiger | Nov 22, 2006 09:59am | #8

          "my take is that OSB is 2-3 times heaver "

          I would have said the same.  That's a pretty interesting article link posted above.  U-Mass says that 7/16"OSB is actually 2# lighter than 1/2" plywood (while OSB is 10# or 14% heavier than the equivalent Sturd-I-Floor plywood).  Learn something new every day!

          1. alrightythen | Nov 22, 2006 01:34pm | #9

             

            "my take is that OSB is 2-3 times heaver "

            "I would have said the same." 

            I would  not have said the same. big difference between 14%  heavier and 100% to 200% heavier.

            but heres one for you....did you ever find it felt lighter to carry 2 sheets of 1/2" ply at same time as opposed to 1 sheet 1" ply? whats up with that? I figure must be the extra glue.

            Edited 11/22/2006 5:35 am ET by alrightythen

          2. wrudiger | Nov 22, 2006 07:41pm | #11

            Yep, big difference between 14% and 100% or more.  It's that perception thing... 

            I have actually found the opposite - the 1 1/8" t&g subfloor ply seems lighter than 2 sheets of 1/2".  Maybe it's the fat core of balsa wood, or whatever the heck they put in it these days.  I know that it used to be heaiver!  Never have put any of it on a scale though. 

            By the way, have you noticed how heavy plywood and OSB get in the afternoon?  What's with that?  Soaking up humidity?

  4. joeh | Nov 22, 2006 07:33am | #7

    OSB will be ok.

    It's not going to collapse on you, if it deflects under a load so what?

    Spend the $ on beer.

    Joe H

  5. ksan | Nov 22, 2006 07:35pm | #10

    I don't claim to be so knowledgeable and sometimes I mix up all these letters--but here is another consideration--how "green" is each material?  Does OSB have VOC?  Formaldehyde? 

  6. vinniegoombatz | Nov 22, 2006 09:28pm | #12

     

    use the ply    if osb absorbs water, from air or a leak, it swells   ply iz better strength between joists and is pretty cheap right now  wuz in a HD and 4 ply (fir?) was $13.50 a sheet   

     

    1. User avater
      Matt | Nov 23, 2006 04:14am | #14

      I just bought some 3/4" T&G OSB today for an attic floor for around $13.50 a sheet.  Not sure what thickness your "4 ply" is but I'm guessing 1/2".  If that is the case, personally, I'd go with the 3/4" OSB without a second thought, especially since the C to C span is 24".

      Edited 11/23/2006 7:30 am ET by Matt

  7. hasbeen | Nov 23, 2006 01:10am | #13

    I assume another route has been taken, but why is there no insulation on your attic floor?

    Just making sure...

    "Doubt is not a pleasant condition, but certainty is absurd."

    ~ Voltaire

    1. CCI | Nov 24, 2006 04:46am | #15

      Thanks to all for the advice - it seems to be a toss up.

      This is an unfinished attic space and this floor is actually going on top of the "joists" that are tying the rafters together.  THe roof is a 12/12 pitch and the "joists" were installed to pass inspection.  I am on long island, ny and the code says no finished space on the third floor.  If the joists are placed at 7ft or lower it is unfinished space.  From the top of the joists (they are too low on the rafters to be considered collar ties) to the bottom of the ridge beam is about 5-1/2ft.  I figured if  I put down a floor and put in a pull down staircase I could make use of this "free" storage space.

      The floor of the attic is loaded with insulation and I can live with a 7ft high ceiling so the easiest thing for me was to install some sheets of whatever onto the top of the joists.

      I think I may stick with the plywood since I am old fashioned and I know it will work.  I guess saving like $50.00 isn't worth the experimentation.

      I will save the OSB for my next sheathing project.

      Thanks again and enjoy Thanksgiving.

      1. oldbeachbum | Nov 24, 2006 07:31am | #16

        4 ply, huh?  Is that enough for future storage and moving about?  Isn't that about 7/16ths or less?  Just ???

        A few $ ahead now or regret later....The unspoken word is capital. We can invest it or we can squander it.  -Mark Twain...

        Be kind to your children....they will choose your nursing home.

         

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