GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Flat roof assembly in a hot humid climate

ablondguy | Posted in GBA Pro Help on

We are going to construct a very modern home with low pitched roofs (1/4″ per foot on 5 separate roof levels) being constructed of 18″ trusses, utilizing Advantech sheathing for the deck, then a vapor permeable fluid or adhered membrane, then rigid Polyiso board varying from a minimum 1″ thick to 6″ to create the slopes, then Densdeck Duraguard and finally a fully adhered, highly reflective PVC membrane. Underneath the deck, we intend to use 6-8″ of open cell spray foam.

This will leave an air gap of 10-12″ between the top of the dry wall on the ceiling and the bottom of the spray foam insulation below the deck. Will this avoid/decrease the likelihood of condensation issues? Would you advise filling the 10-12″ air gap with insulation or is that over-kill?

We haven’t been able to access some of your prior articles due to website issues (have notified customer service) and have seen some examples from Joseph Lstiburek that show this gap open and some that show it filled depending on the website, and without regards to what climate zone.

Please advise.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Six inches of open-cell spray foam will give you an R-value of about R-22. One inch of polyiso (the thinnest layer of your above-sheathing insulation) will give you about R-6. If you add the two together, that's about R-28. That is less than the minimum code requirement everywhere in the country.

    However, it's close to the minimum (R-30) in Climate Zones 1, 2, and 3.

    Assuming you are able to hit the minimum code requirement for ceiling R-value, there is nothing wrong with having an air gap between your ceiling drywall and the underside of your open-cell spray foam.

    For more information on these issues, see Insulating Low-Slope Residential Roofs.

  2. ablondguy | | #2

    "Thanks Martin! We will definitely increase to at least 8" of open cell and possibly increase the minimum thickness of the Polyiso to 2" - if possible at the drains with low parapet concerns."

  3. Expert Member
    ARMANDO COBO | | #3

    Norman – I’ve taken a different approach to flat roof drainage in the SW for many years. I do not design drains because they can freeze and get plugged-up with debris so the maintenance issue is high. I rather do “canales” or scuppers with the roof slopped to one side. If the truss is designed with ¼” slope, you don’t need to increase the slope with insulation; it’s too expensive. I would rather increase the slope in the trusses to ½” and just install the “flat” rigid foam over the decking. The framer needs to build crickets to the scuppers, and make sure they are not draining to a door or window.

  4. user-869687 | | #4

    Norman, I fully agree with Armando here. First, increase the slope to 1/2", because when you spec 1/4" you are asking for standing water on the roof. Look around at rooftops and you'll see plenty of low slope roofs with standing water after rainfall. Those roofs were spec'd at 1/4" slope but Murphy's law and/or entropy got the better of them. Second, drain to one side through the parapet rather than to an interior drain. Third, slope the OSB deck, and then your polyiso can be a constant thickness.

  5. dmatranga | | #5

    I have the same issue as Norman (original poster). Building a modern home in Houston with a flat roof, and I plan on taking Armando's advice with regard to requesting that the truss manufacturer slope the trusses by 1/2" per foot.

    TJ, what do you mean by slope the OSB deck? Would you have to cut 2x's and put them under the decking? Just not sure how you would advise sloping the OSB part.


  6. GBA Editor
    Martin Holladay | | #6

    There are several ways to make sure that your OSB roof sheathing has a slope.

    One way is to install the rafters at a slope. This is the most common method.

    Another way is to specify special trusses with a horizontal bottom chord and a sloped top chord.

    A third way would be to install your rafters dead flat and then to shim the tops of the rafters. This last method isn't very logical, however.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |