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Opinions on this Plan for Roof Insulation and Venting

bjh85 | Posted in General Questions on

I purchased a very old house, and I want to completely replace the 24’x5′ flat roof on its side. The 24′ dimension runs along my house. The 5′ joists are merely 2x4s(!!), and there wasn’t even insulation in there! The slope is 0.25/12 and it’s covered in EPDM.

Here’s what I would like to do:
– replace 2x4s with either 2x6s or 2x8s spaced at 24″
– increase the slope from 0.25/12 to 3/12
– add 7″ of EPS (I have sheets of 3.5″ EPS on hand) between the joists using the “cut and cobble” method (
– add 2×4 strapping perpendicular to the joists for venting from the “short side” of the the roof (4 rows of strapping, so 60″/3 => 20″ on center between the 2x4s)
– 60 mil black EPDM, but open to the idea of shingles

Here are some questions that come to mind:
– 2x6s or 2x8s?
– is EPS ok for this purpose? In Minnesota, would a vapor barrier below that still be necessary assuming spray foam used to seal EPS to joists?
– thoughts on the venting? Alternatively I could skip the strapping and vent on the “long side” (ie each joist bay) by leaving a gap between the sheathing and the EPS. However hot air would not have an exit.
– I have some widows above this roofline. What’s a safe distance between the top of the roof and the bottom of a window?
– If I do a 3/12 pitch, then I’ll have a triangular space below the joists and above the drywall that’s approximately 5’x15″. Should I stuff this with fiberglass (which I already have) or leave empty?
– If I am going to put the 2×4 strapping over the joists, does code ever allow for spans longer than 24″ between joists? (Clearly I’d have to check with local code.) If so, that would reduce thermal bridging. I’m skeptical they’d allow it.

I appreciate everyone’s feedback!

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  1. user-2310254 | | #1

    Reclaimed roofing foam is readily available and much less expensive than new product. And the cut and cobble is generally not recommended since the caulked seams tend to open up over time.

    What about buying reclaimed and tapered roofing foam instead? Do you have room to build up the shed roof on the exterior?

    1. bjh85 | | #2

      I had been planning to seal the EPS to the joists using cans of spray foam. Do those also open over time? If so, I didn't realize that.

      I do have roughly 20" of vertical room on the exterior (before I hit my window). One reason I want a complete overhaul is to be able to better insulate it.

    2. kentthompson | | #4

      Reclaimed foam is not easily found (at least by me) in northern California. It might vary by region, but great suggestion to look for it regardless.

      1. GBA Editor
        Kiley Jacques | | #5

        Kent, try searching the term “rigid insulation” on Craigslist, or contact a specialty supplier like Insulation Depot.

        1. kentthompson | | #8

          CL doesn't have anybody selling and Insulation Depot turned out to be more expensive than new insulation with shipping to me in Sacramento from Reno. I called a bunch of roofer demo folks to see if they could work with me directly but no dice. Maybe other folks have better luck but that was my experience.

  2. Expert Member
    Akos | | #3

    A simpler solution is to install the rigid directly above the existing EPDM. Install as much as you want/can (4" to 6" depending on climate) and cover with a new EPDM membrane. Make sure to flash the membrane properly at the wall transition, this is always the weak spot especially in snow country. You can also get a small R value bump by insulating the 2x4 ceiling cavity with batts. This is an unvented assembly so make sure to seal any roof/soffit vents in this section.

    If you want a bit more slope for better drainage, you can get tapered polyiso from a roofing supply place. Bumping up the slope all the way to 3/12 might cause more problems than it solves especially if you get close to your windows.

  3. GBA Editor
    Kiley Jacques | | #6

    If you decide to go the unvented assembly route, this detail might be useful: Unvented Roof for a Cold Climate

  4. walta100 | | #7

    Your house has stood the test of time and today’s code would not likely allow today’s 2x4s to support that roof. Generally the old timers often did things the way they did for good reason.

    If the roof is flat they likely had and may well still be a good reason to have a flat roof.

    EPDM was not an option in the old days so some stuff has been changed lately.

    Understand an unvented low pitched 3-12 roof stuffed full of insulation could easily turn into a moldy rotting mess if you ate not careful.


  5. bjh85 | | #9

    I ended up going in a different direction with this project. I decided that I wanted to retain the flat roof since it's a roof deck. I replaced the 2x4 joists with 2x6 joists that are spaced 16". These 2x6 joists are ripped on a slight angle so that the resulting roof is sloped .25" every 12". In other words, the joists are 5.5" tall on one side and 4.25" tall on the other side. 3/4" plywood is attached directly to these joists. Above the plywood is 1/2" fiberboard and 60-mil EPDM.

    Now I want to insulate, which is where I could benefit from your collective expertise. I already own used 3.5" EPS sheets that I'd like to "cut and cobble" between the joists (using cans of spray foam to seal). I plan to vent each joist bay, so I'd leave a gap between the plywood and the EPS. 2 questions: Any reason not to do this? If so, what do you think is a better plan? Should a vapor barrier be used below the EPS?

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