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Roof Decking

Teeds | Posted in Construction Techniques on July 20, 2002 11:58am

I am going to start a apartment renovation project in Indianapolis soon and ran across something that I had never seen before.

The roof decking is 3/8 inch plywood and installed over trusses at 24 inches o.c.  The roof felt soft and in numerous cases it cracked as I walked across it.  I am not sure of the slope, but would guess 3 on 12.

Local roofers indicated that this was common practice for the area when the project was constructed (1972) and is still used frequently.  One roofer did indicate that he would have to remove the sheathing and replace it in order to warranty the roof.  His suggestion was 7/16 inch OSB!

I am from Texas, and down here it is common practice to use 5/8 inch OSB with supports at 24 inches o.c?

Any feedback would be greatly appreciated.

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Replies

  1. User avater
    JDRHI | Jul 21, 2002 12:30am | #1

    Here in my neck-o-the woods we dont use trusses much except for prefabbed. That said, I also dont come across 24" centers all too often either. IMHO, I would recomend the beafiest sheathing available. I'm surprised the truss spacing didnt telegraph right through the roof itself.

    J. D. Reynolds

    Home Improvements

    "DO IT RIGHT, DO IT ONCE"

    1. Teeds | Jul 21, 2002 01:01am | #2

      In reality it did telegraph through.  Also, the roof framing is trusses.  One roofer asked about whether or not we knew if the decking was laid using clips.  Based upon my observations noted while walking the roofs, clips would not have mattered, and for the life of me I cannot remember ever seeing clips for 3/8 sheathing.

      I agree on going with the thicker sheathing, but .... you know what that means, a thinner wallet for the owner.  I wanted to get some outside opinions before I spoke with the owners.

      What about the comment about 7/16 inch sheathing.  That still seams thin to me.  I know that hurricanes drive the requirements along the coast, but I am amazed that 1/2 inch isn't the minimum and 5/8 inch the preference.

      Tony

      1. rez | Jul 21, 2002 05:10am | #3

        3/4 inch osb tongue and groove subfloor sheathing is $9.99 a sheet here. Won't this work well for roofing also?Half of good living is staying out of bad situations.

  2. User avater
    DaveMason2 | Jul 21, 2002 10:37am | #4

    I would price the 7/16 OSB with clips and the 5/8s without and then decide which one to use. If the prices are even remotely close I would use the 5/8s.

      

     Dave

    1. Teeds | Jul 21, 2002 03:09pm | #5

      Dave - That is a really good idea!

      I am writing my specs today and getting them out to roofers.  I will ask for pricing on both deck systems.  I will be interesting to see what the response is.

      I'll report back with the results.

      1. johnnysawzall | Jul 21, 2002 05:26pm | #6

        How much snow do you get in ind.? For you Texans, thats the white stuff that floats down from the sky, is very wet, and will lay very heavy on a 3/12 roof @24" centers! I use trusses often here in Pa. and very often I use 7/16 osb on 24" centers. (always with plywood clips, yes they do work) but I have never done this on less then a 6/12 pitch. Anything less I would go 16' o.c. or beef up the plywood.                                                       "God bless Texas"

      2. Piffin | Jul 21, 2002 05:54pm | #7

        A lot of your concern seems to be in the 7/16" dimension, Tony. This product is technically supposed to be comparable structurally to the 1/2" plywood in similar applications. If you get the search feature here to work for you, you'll find that we've had a couple loooooooonnnngg arguements about whether this is true or not, focusing somewhat on snowloads and whether adequate ventilation keeps humidity and heat from weakening the osb.

        I'm in the camp that refuses to use osb on roofs but will use it for wall sheathing. 5/8" might be acceptable with clips though. I've seen lots of 7/16"osb telegraph through.

        I think yopu need to know the existing condition of the 3/8" ply before writing specs. If delaminating and rotting, it should be removed - otherwise, it should be ok to add the new sheathing over it. This is a big factor in the cost. Openning up to bare trusses can loosen the structure and lead to sheetrock damage on ceilings inside too. As a roofing contractor, I might want an open clause refering to that fact....

        Your original situation is unique. I've only seen 3/8" on mobile homes and DIY camps.Excellence is its own reward!

        1. Teeds | Jul 21, 2002 09:52pm | #8

          White fluffy stuff coming down from the sky ???? Last time I remember that stuff was when I walked to and from school uphill both ways barefoot ....

          Seriously, in two cases, the 3/8 inch plywood appears to be delaminating, judging by the cracking as I walked across it.  At the very least, there is no edge support or clips between the sheets.

          I guess I am focusing on the thickness of the plywood, as it has clearly failed in at least two cases.  Oddly enough, the two buildings with the worst substrate are orientated N/S while the buildings orientated E/W appear to have damage limited primarily to the south face.

          These apartments are low income housing that has had little or no preventative maintenance over the life of the project.  This, and the fact that they were constructed as low income housing has contributed to the situation.  There have been many corners cut over the years.  My guess is that the roofs are original which puts them 10 to 15 years past any reasonable life expectancy.  I had never seen decking this thin utilized and was wondering if I was unique in this knowledge gap.

          I have rarely used anything except 3/4 inch plywood (more often than not T&G) on roofs here in Texas.  I can't say that it is because of hail, because I have seen hail come through 3/4 inch deck and not slow down until it his the concrete slab.

          One roofer has indicated that he will not overlay the existing deck with new decking.  He indicated that he must remove it.  The other roofer has been moot to this point.

          Thanks for all the knowledgeable input, it is greatly appreciated!

          1. Piffin | Jul 21, 2002 11:50pm | #9

            The guy who wants to remove it is my choice, for sure. It's just one indication that he is intrested in a quality job for you.

            When I started roofing in Florida in 1970, we were doing a lot of low income and govt subsidized aptartment buldings. I questioned (this was back when I really didn't know very much at all) some of the qulaity and methods I saw and was told that this was designed so as to return money to the investors within less than ten years and that there was no concern for ongoing maintainance or longevity and an assumption that they would be torn down withing twenty years or so....Excellence is its own reward!

          2. rez | Jul 22, 2002 06:13am | #10

            It just blows me away that you guys are roofing 7/16inch osb on 2 foot centers. If you'll put up with these prodding questions from a DIY to the pros it would be appeciated in that I'm always willing to learn. I am wondering what size nails one would use and does the 7/16osb  hold that nail well enough?Half of good living is staying out of bad situations.

          3. heck22 | Jul 22, 2002 06:22am | #11

            I haven't used anything less than 5/8" on a roof deck of any type in over 20 years.

            Have not ever had a sheathing-related failure.I know what that is worth to me.I expect that my customers were well served for the small difference in price.

            JWwhat the heck was I thinking?

          4. rickchem | Jul 23, 2002 09:05pm | #13

            Unfamiliar with 3/8???  I'm no roofer, but in the mid-atlantic area, that seemed standard w/ 24" OC trusses, with clips, in the 70's at least.  Yes, the trusses telegraph through, and the roof gets kind of wavy after 20+ years.  But that was the way they did it!  Never seen a catasrophic failure - though it does not inspire confidence walking across it.  1/2 or 7/16 OSB seems standard now.

                I'm going to replace some panels in a 3/8 roof . . . I'm guessing I am going to need 7/16 ply to match the 3/8 so the height difference is not noticeable. . . right?

          5. Piffin | Jul 24, 2002 03:32am | #14

            Well now, don't forget that the 7/16" swells up to at least 1/2".

            LOL

            Now I'm wondering if the load ratings automatically increase as the thickness does.

            ;)

            Excellence is its own reward!

  3. User avater
    BossHog | Jul 23, 2002 06:46pm | #12

    I haven't seen a new roof go in with 3/8 plywood for 20 years or better. But I guess it was common practice for a while. I can show you a building with 3/8 plywood and trusses set 32" O.C. near here. Believe it or not, it's the main building for a truss plant.

    I don't really like 7/16 OSB for roof sheathing. But I can't show you a roof where the OSB has sagged so bad that it ought to be replaced. But it's nail holding capacity bothers me.

    Going to 5/8 seems like it's almost too much. But 1/2" plywood seems reasonable. I'd say that 90% of the roofs around here are done with 1/2" OSB or plywood. The majority of the rest are done with 5/8, and just a few with 7/16.

    I once helped a DIY (relative) do his roof with 3/4 T&G OSB. That was absolutely nuts, but it's what he wanted. Sure was a pain, though.

    bumpersticker: Practice safe sex. Go screw yourself.

    1. User avater
      JDRHI | Jul 24, 2002 04:49am | #15

      I used 3/4" ply when I replaced my roof. 2x8 rafters 24"OC w/out a ridge. I left the 3/4" batten strips from the original cedar roof and screwed the ply right through to the rafters. My house is 96 yrs old and the roof "apears" perfectly flat. Sometimes overkill just gives me a sense of security.J. D. Reynolds

      Home Improvements

      "DO IT RIGHT, DO IT ONCE"

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