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Roof insulation question

thesouth18 | Posted in Green Building Techniques on

Hello everyone. I live in an 133 year old home that “breaths”  lol.  I’ve been sealing up as much as I can, and I’m currently going to reframe an addition that was added on in the 1970s because unlike the 133-year-old portion that is still in great shape, the 1970’s workmanship isn’t.

I want to utilize ZIP-R on the walls.  The trick is the roof.  We would like to put some decking on the roof to walk out on from our second story because the current roof there is almost flat with a membrane.  Is there any type of exterior foam I can put on that would hold up to a deck?  I won’t be installing a hot tub or anything, just some railings and people.

Any ideas would be great!  Thanks so much!

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to build a deck above a flat roof, you won't be walking on the roofing (the membrane). You'll be walking on a wood deck (or in some cases, concrete pavers) installed above the roofing membrane. This type of deck generally requires careful detailing to distribute the load, and to provide drainage between the deck and the roofing membrane.

    If these ideas are new to you, you'll probably want to hire a contractor to design and install your deck.

    (By the way, when roofing membrane is installed over rigid foam, roofers often include a so-called "coverboard" between the rigid foam and the roofing membrane. The roofing membrane usually doesn't go directly on top of the rigid foam.)

  2. thesouth18 | | #2

    Thanks Martin, yes I understand I would not be walking on the roof proper. There is a system by Bison that is a pedestal/wood tile that I would use on top. My concern is if the external insulation would be able to hold the weight and not compress/fail. I've had a structural engineer calculate the framing I'd need so that has been taken care of.

    1. Expert Member
      Akos | | #4

      Most common foams are around 25PSI compressive strength. So one sqft of foam can support 3600lb without a factor of safety. With 8" base pedestal that is about 1200lb.

      This is just rough hand calcs, but I doubt your deck would deform the foam. Best to check with your engineer none the less.

  3. Expert Member
    Peter Yost | | #3

    Hi South -

    Seems as though if you engaged an structural engineer for calcs on the framing you would ask him or her about what structural properties you need from your insulation (compression and spreading load)?


  4. thesouth18 | | #5

    I did not even think about that at the time or know it was a possibility. I was thinking of spraying under the deck with closed cell when I had the calculations done. Now that I know it's even possible, I can ask him. Thanks!

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