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

Comments on unvented flat roof detail

adb2020 | Posted in GBA Pro Help on

I’m in the permitting phase of a workshop addition (conditioned space) in Climate Zone 3C (SF Bay Area, WUI) and my architect has produced the attached unvented roof assembly detail.  The slope is not drawn well but will be o.25″ per foot.  Joists are 2×12 LVL.  Project is subject to 2016 CBC.  Lastly, we need to rough in fire sprinklers within the joist space.

I had proposed rigid insulation sandwiched between plywood sheathing but that apparently presents issues for the waterproofing membrane.  So, spray foam was recommended.  My understanding is that closed cell may be a safer option for flat roofs, but I’m hoping to confirm open cell is acceptable given the waterproofing membrane. 

Also, with the R-value requirements being relatively low here and ample joist depth, should I consider using a combination of spray foam and air-permeable insulation to reduce costs?  Am I correct that code requires closed cell when using the flash and batt method?

Thank you!

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. Expert Member
    ARMANDO COBO | | #1

    I like to specify 1"R6.7 min. or 2" R13 polyiso on top of the roof decking, cover board and TPO on top. I would install cellulose under cavity bellow the roof decking.

  2. etekberg | | #2

    Don't know jack about it. But pretty sure Lstiburek does :

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |