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Will my roof sag if my exterior roof insulation is too think? Rockwool (comfort board 80) & 5/8″ Zip Sheathing

jrosucci | Posted in Green Products and Materials on

We are trying to figure out how thick we can go with our exterior roof insulation and not have our roof sheathing sag (from compression of the rockwool).  
We are building a single level home with a 4/12 pitch on gable roofs with comp shingle over a conditioned attic.  We are applying the sandwich technique and using 5/8″ zip sheathing on the trusses, Comfort Board 80 for exterior roof insulation on top of that, and 5/8 zip on top of that to complete our roofing structure and insulation.  We are in zone 3C and can use 1.5″ to meet our requirements, but ideally I would like to use 3″.  That is of course, as long as I don’t get a saggy roof field.  I would prefer not to use sleepers in the middle of the field to keep the roof rigid, but am open to ideas from those that have dealt with this before.  

Hopefully I didn’t leave out a pertinent information.  Thank you for reading my quandary.

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  1. jollygreenshortguy | | #1

    Comfortboard weighs 8 pounds per cubic foot, or 2 pounds for a 3" thickness. An extra layer of OSB adds another 2 psf. Unless your roof is already grossly under-built I don't see how an additional 4 psf could be a problem.

    If you haven't built the roof yet just account for the extra weight in the design as part of the dead load. If you're using prefab trusses this is routine.

  2. gusfhb | | #2

    Tell your truss company you are adding 4 psf they can figure it out.

  3. Expert Member
    Michael Maines | | #3

    Here in snow country, we design roofs for anywhere from 50 to 110 pounds per square foot, so an extra few pounds would not matter. In your climate zone, you have much lower design loads. As Gus says, just tell your truss company about the extra weight and they can easily accommodate it.

  4. Expert Member


    The responses you have been getting have been about the additional weight of the insulation, but I'm unclear as to whether you are asking about that, or the possibility of the thick mineral wool compressing. Can you confirm what you are worried about?

    1. Expert Member
      Michael Maines | | #5

      I did miss that element, good point. I haven't tried to install sheathing directly over mineral wool but I wouldn't recommend it; installing strapping or furring first allows you to create a level plane that I'm quite sure would be much more challenging to do with sheathing alone.

    2. jrosucci | | #6

      Yes Malcom, I now see what I forgot to mention in my original question. I have since added to the original question that my concern is about the compression of the rockwool insulation and a sag in my roof field. Thank you all that have responded.

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