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

Spongy flat roof

Timothy Robinson | Posted in GBA Pro Help on

We recently bought a house, in Alaska, that a commercial concrete contractor and his contractor friends built in stages as he, the cement contractor, lived in during the various stage of completion.

The roof is absolutely flat is built with 14″ BCI that are 24″ on center. He had closed cell foam sprayed under the roof deck, and added 2 layers of 5″ x 24″ fiberglass batts. There is no vapor barrier on the underside The roof sheeting is 3/4″ CDX with a hot mop tar and paper coating. Needless to say there is no drain and thee are no venting of the roof deck and it has gotten soft in a number of places.

I am contemplating sheeting over the soft deck and building a 1/12 shed roof with an EPDM cover over the existing decking, ripping out all of the ceiling drywall and re-mediating any problems that may be present. Should I insulate the new shed roof and add vapor barrier between the new drywall and old roof deck?

Thank you in advance and I look forward to your help.

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

    When I read your question, I had no idea what "BCI" stood for. After performing some research, I learned that it is a brand name for I-joists manufactured by Boise Cascade.

    Here's my advice:

    1. If you have spongy roof sheathing, you have to remove the roof sheathing before you install new materials above the roof sheathing. You don't want to put new sheathing on top of spongy sheathing.

    2. Before coming up with a repair plan, it is essential that you understand why this roof failed. You didn't mention the thickness of the spray foam layer, but my guess is that one of the reasons for failure was that the spray foam layer was too thin to prevent moisture accumulation during the winter. For more information on the ratio rules that need to be followed when you combine foam insulation with fluffy (batt) insulation, see this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    Although the article I linked to talks about rigid foam insulation, not closed-cell spray foam insulation, the same ratio rules apply to roofs that combine spray foam and batts.

    3. You don't explain how you will go about "building a new 1-in-12 shed roof over the existing decking." Do you plan to install new sloped rafters? Or do you plan to install tapered rigid foam? Whatever you do, get rid of the spongy sheathing before you start.

    4. Assuming that you are planning to install new sloped roof framing above the existing roof, and that you will pull out all the old insulation materials from the inside, you need to follow the advice given in this article: Insulating Low-Slope Residential Roofs.

    -- Martin Holladay

  2. Timothy Robinson | | #2


    Thank you for your response. The foam was sprayed at various thicknesses sometimes 4" sometimes 2 inches from my inspection thus far. The roof is an absolute hot roof,
    Recently I built two vaulted ceiling roofs based on an article you post previously using the CCHRC method you suggested previously. It was a tedious method but seems to be working well!

    The current roof also failed due to the amount of snow, melting snow and ice sitting op top of a roof with no slope or the ensuing moisture created.

    My intentions are to isolate the existing roof and build a roof above it ,4 to 5 feet on one end and planing into the four foot overhang at the other end. I would vent the new roof. where there would be an air space between the new roof and old. Again, my question is should I use a vapor barrier on the new interior ceiling? The i-joists overhang each side of the existing roof by four feet.

  3. Timothy Robinson | | #3

    I intend to insulate the new shed roof with the appropriate method.

  4. GBA Editor
    Martin Holladay | | #4

    In your climate zone, building codes generally require a vapor retarder, not a vapor barrier, on the interior side of vented roof assemblies. In Alaska, almost everyone uses interior polyethylene -- and that approach works well in Alaska.

    That said, an interior vapor retarder is much less important than an interior air barrier. Pay attention to airtightness at every stage of construction if you want your roof assembly to perform well.

    -- Martin Holladay

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |