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Discussion Forum

Roof decking – OSB vs. Plank

user-72068 | Posted in Construction Techniques on July 23, 2007 11:13am

I’m building the only home I plan to live in for the next 30 years on the family farm.  I want a roof deck that will be strong and help in control my HVAC costs associated with the hot and very humid west <!—-><!—-> <!—->Tennessee<!—-><!—-> summers.  My current insulation plan is to use 1†rigid foil-faced polyiso on the exterior, 2†of sprayed foam and cellulose in the 2 x 6 wall cavities.  Given the hot, humid <!—-><!—->Tennessee<!—-> <!—-> summers and falls that we have, here are my two roofing deck options:

<!—-><!—-> <!—->

1)      Deck with 1†x 12†planks.  My GC’s favorite option; he indicates it will be stronger than an OSB deck and that I should just add more insulation in the attic.

2)      To help combat attic heat gain and cooling costs in the summer, deck with foil-faced OSB panels (19/32†x 4’ x 8’) like LP TechShield or Eclipse radiant barriers.  

<!—->  <!—->

If you were building your last home on the family farm, would you go with the plank roofing deck and just blow in more attic insulation?

<!—->  <!—->

Or would you go with the radiant barrier approach on the roof deck?    

<!—->  <!—->

My GC is a great guy with 25+ years of experience and he’ll do the foil-faced OSB if I request it.  My question is, what would you do in this situation?  I’m open to all comments, feedback and suggestions.

<!—->  <!—->

Thanks, Stan

 

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  1. jesse | Jul 23, 2007 11:45pm | #1

    If you are spraying foam in the walls, why not the roof, too?

  2. renosteinke | Jul 24, 2007 01:01am | #2

    OSB, of a similar thickness, ought to be every bit as strong as pine boards .... without the warping, cupping, twisting, and splitting that today's lumber seems to excel in.
    OSB is harder to get up on the roof ... here the secret is to have the supplier use a crane ... and harder to staple / nail into. Use of pneumatic nailers is almost mandatory.

    Do not confuse OSB with chip board, flake board, particle board, or any of the earlier types of 'manufactured' product. Things have come a long way since then.

    Also, be sure to look up, and follow installation instructions - especially regarding spacing between boards.

    I have not seen wood planks used on roofs in decades. I think OSB is perfectly acceptable.

  3. DanH | Jul 24, 2007 01:10am | #3

    Actually, my choice would be 1-foot-wide T&G Advantech-type planks, but I don't know that anyone makes such a thing.

    So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
  4. Piffin | Jul 24, 2007 02:51am | #4

    I would use the planking and spray foam to the backside of it

     

     

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  5. RedfordHenry | Jul 24, 2007 03:32am | #5

    Foil faced OSB and as much insulation as the attic will hold.  Your ventilation plan is far more critical to managing heat build up and moisture than your choice of roof deck.  Negligable thermal differences between the OSB and planks.

  6. ampres | Jul 24, 2007 03:54am | #6

    I think either would work fine, just a matter of personal preference.  The 1 x 12's are old school and would probably cost more for material.  OSB is fine as long as it's installed correctly.  I personally would use Advantech sheathing.  Spray foam on the back of the sheathing.  I've never used their Zip system but would certainly consider it.

    I have no experience w/ LP Techshield but it looks like an interesting product.  Their website claims it will reflect 97% of the radiant heat in the panel.  If it actually works then it may be the way to go if you are concerned about heat, although I haven't heard anything on how well it works.  It doesn't look like you'd be able to spray foam on the back of this sheathing either.

    Just out of couriosity, why 2x6 walls if you're only spraying 2" of foam in the walls?

    1. user-72068 | Jul 24, 2007 04:11am | #7

      I'm planning on having a "flash spray" of closed-cell foam to seal everything and then fill up the remainder of the 2 x 6 cavity with cellulose.  This method was highly touted by the first insulation contractor (IC) I spoke with.  The IC stated that 6" of spray foam was "overkill" in a 2 x 6" wall cavity.

      Personally, I thought why not have ~ 6" of spray foam insulation.  I plan to get second and third opinions (and quotes).

      Thanks for the input!

      1. Piffin | Jul 24, 2007 05:07am | #8

        Yes, you need to study insulation more!Better to frame with 2x4 and spary foam 3"Study total spray. orbond's "Walls that work"
        Pay attention topreventing infiltraation at sills and at roof/cieling. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

      2. ampres | Jul 24, 2007 11:31am | #9

        Ditto what Piffin said.  2x4 walls foam only forget the cellulose.  6" of foam would probably be overkill.  I'm guessing closed cell foam, around 6 - 7 R per inch which would get you around R 36 in a 2x6 wall filled w/ foam.

        Looking at this a second time I would say that if you are going through the expense of spray foam in the walls (well worth the money IMO) then you would be foolish not to foam the roof deck as well.  This will eliminate having to vent the attic which would then become conditioned space which would then in turn eliminate common problems with HVAC equipment and ductwork in the attic along with other common attic insulation issues.  This would probably cause you to not be able to use the LP foil faced OSB you mentioned since their website stated insulation cannot be in contact with the foil side.  They want a 3/4" air gap between the foil and the insulation.  You would want to research this further, maybe they only mean FG or cellulose.  Perhaps give them a call to see how they recommend installation with spray foam insulation.

         

        Edited 7/24/2007 4:50 am ET by ampres

        1. Piffin | Jul 24, 2007 02:13pm | #12

          "if you are going through the expense of spray foam in the walls (well worth the money IMO) then you would be foolish not to foam the roof deck as well"That's my point about infiltration 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

  7. Hazlett | Jul 24, 2007 01:40pm | #10

     Stan,

     your roofing material choice is going to come into play with this decision.

     Personally--- i prefer solid 1x decking on rafters 16" O.C.------------

     however----if you are planning to roof with asphalt shingles---in particular heavy weight dimensional shingles-----you are going to want to adjust your plan to use 1x12's

    Certainteed will recommend 1x6's maximum------although I install on SYP 1x8's every day.

     the problem is a combination of 2 forces at work

    1) heavy weight laminate shingles have VERY aggresive seal down strips-----gluing the whole roof surface together so it moves more like one BIG sheet

    2)1x12 will have a lot of movement across the grain

     the potential problem is that the movement will actually split the shingles.

    reducing the size of the 1x12 to 1x6 or 1x8 will minimize the potential movement and reduce the liklihood of wide spread splitting in the shingles

    BTW--before anyone points out that roofs were shingled with asphalt shingles for decades over 1x12's---that was before sealdown strips, modern laminate shingles and their  highly effective adhesive strips.

    stephen

    1. DanH | Jul 24, 2007 02:09pm | #11

      > the potential problem is that the movement will actually split the shingles.This is why I think hardboard roof sheathing should be made in narrow planks.
      So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

      1. Hazlett | Jul 24, 2007 02:55pm | #13

         I don't know about that line of reasoning Dan------full size 4x8 panels are pretty stiff-----but when I occasionally rip 3/4 plywood or ( heaven forbid) osb down to patch in  a space formerly occupied by a solid 1x8------the 1x8 piece of plywood or osb is pretty floppy-----compared to the rigidity of a solid 1x8

         stephen

        1. DanH | Jul 24, 2007 04:27pm | #14

          But four planks instead of one panel would distribute the stress over four joints vs one. And if the planks were T&G they'd be quite solid, with fewer problems at vertical joints.
          So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin

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