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Community and Q&A

Low-slope roof

Jon_Lawrence | Posted in Energy Efficiency and Durability on

I am building a PGH and I have a 32′ run of unvented, low-slope roof that we will start framing out shortly. The interior stack-up is gypsum, 2×3 service cavity, Siga Majrex air and vapor control layer (let’s vapor out of the cavity, but not in), 2×12 joists, dense pack cellulose between joists, 3/4″ Advantech seams taped with Wigluv. I was planning on using a unvented nailbase product that has 4.5″ of polyiso with a layer 1/2″ osb attached to the top. I am in Zone 4 and this will provide more than the required .31% of exterior to total r-value even if I de-rate the polyiso r-value to 5. I am trying to decide where to create the pitch, above the nailbase or below it. If I put it below, would I have to add a layer of foam that is tapered to match the pitch of my ripped 2×10’s used to create the pitch or can I leave the unvented air space? I have found a local EPS manufacturer who can provide me with the tapered EPS. Alternatively I could place the pitch above the nail base and then the issue becomes trying to secure my 2×10’s to the structure below, although this is somewhat mitigated by the fact that the ceiling joists below are doubled-up every other bay so I have a wider area to try and hit with a screw.

Btw, the purpose of this low-sloped roof is to provide a place for solar panels that cannot be seen from the street, which is why it was engineered with the extra joists.


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  1. Expert Member
    ARMANDO COBO | | #1

    I hope I understand your question, but I can't tell if you are using 2x12s or 2x10s for joists, but either way, that depends on your span and load capacity, specially with PV on top. Typically, it is easier to create a sloped roof with same height walls by installing mono-sloped trusses. Any roof built to install PV arrays need to have additional 6-10 lbs dead load in the structural design. Personally, I'm more comfortable with doing an unventilated assembly.
    You don't mention roofing material, but with flat/low-slope roofs, best is to use TPO or metal roofs. Living in NJ, your panels will have highest efficiency set at ±40°incline, therefore, your mounting system needs to be coordinated with your roof built-up and material. Standing seam roofs are easier and less expensive to install racks.

  2. GBA Editor
    Martin Holladay | | #2

    Armando is correct about the lack of clarity on your framing. At one point you talk about 2x12s; later you refer to "ripped 2x10's used to create the pitch." Which is it?

    However you decide to create the pitch, you need to avoid the creation of any air spaces in the layers between the cellulose insulation below and the foam insulation above.

  3. STEPHEN SHEEHY | | #3

    Regarding Armando's point about optimizing roof pitch for solar production, my real world experience is that the difference roof pitch makes is trivial. Half my roof is 3/12 and half is 8/12. With 23 months of monitoring, the shallower pitch has produced between 2 and 3% less total kwh. We are at about 43° North. One reason is that production is significantly higher on the shallow roof during late Spring and early Summer when the sun is highest and solar production is as well. I get relatively more kwh in winter from the steeper roof, but winter production is low overall.

  4. Jon_Lawrence | | #4


    Thanks for the responses and sorry about the lack of clarity. The 2x12's are the ceiling joists below the Advantech. To be more specific, these are actually LVL's as spec'd by the engineer to carry the snow and PV loads - including a ballast racking system to avoid drilling holes in the roof. The ripped 2x10's would be above the Advantech and used to create the 8" drop in elevation assuming we go 1/4" per foot of run.

    Armando - there is a church with panels mounted on a flat roof about a 1/4 mile from the job site which was the inspiration for putting the PV there. They appear to have a 40 degree inclined as you suggested. The roofing material spec'd is EPDM although my framer suggested metal.

    Martin - I will lay the nailbase directly on top of the Advantech to avoid creating an air space and create the pitch above - how TBD. Now that I know where to put it, I can see what my framer suggests.


  5. Jon_Lawrence | | #5

    I am pretty close to finalizing the details for the roof. I have attached a picture to give everyone a clearer picture of what I am dealing with. There is a long gable in the front and 2 shorter gables on each side. This allows us to place the panels on the flat roof so they won't be seen from the street. This picture was taken around 5pm yesterday so you can see I am still getting quite of bit of sun on the western facing gable.

    I have found tapered EPS and polyiso manufacturers who can provide me with a 9" taper from end to end. Everyone is quoting 20psi. I will have 5" minimum at the roof's edge and at 20psi EPS is coming in with an r-value of 5 which is what I would de-rate the polyiso to. At a minimum r-25 exterior, 42 interior with 12" DP cellulose, I will have 37% minimum rigid to total r-value for the cold edge of CZ 4. The foam will be screwed into the roof joists per the manufacturer's recommendations.

    On top of the foam I will place a 1/4" layer of roof cover board, probably GP's DensDeck. This layer will either be glued or screwed to the foam. I will then use a single-ply membrane as the final layer in the assembly. Per GP's spec's this detail is comparable with both solar panel and green roof applications, both of which I plan on having on this roof.

    One detail I am not sure about is should I place a vapor barrier between the roof OSB sheathing and the foam or not?

    Any comments or questions would be appreciated.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Should I place a vapor barrier between the roof OSB sheathing and the foam or not?"

    A. There is no need for a vapor barrier in that location. (The 5 inches of EPS is already a significant vapor retarder -- not that you need one. Moreover, your membrane roofing is a vapor barrier.)

    What you do need is an air barrier. Taping the OSB panels is probably the best approach. The second-best approach would be taping the EPS panels.

    It's true that your membrane roofing is an air barrier, but there are occasional reports of convective looping in the air pockets between the EPS panels. So an air barrier under the EPS is best.

  7. Jon_Lawrence | | #7

    Thanks Martin.

    You can't tell from the picture, but I taped about half the OSB panels under the tarp so far with Wigluv before I ran out of tape and then wondered if I should just add a vapor barrier instead. I will finish taping and scrap the vapor barrier concept.

    Btw, the gable roof pitch is 8.75/12 and my heels and ankles were hurting after taping those seams. Did you ever get used to it or did it always hurt?

  8. GBA Editor
    Martin Holladay | | #8

    Again, you don't need a vapor barrier. You need an air barrier. If you don't want to tape the seams of the OSB, you could install a European air barrier membrane or a brand of synthetic roofing underlayment that qualifies as an air barrier. If you do that, the seams of the membrane need to be taped.

  9. Jon_Lawrence | | #9


    I think we are on the same page - I am just going to finish taping the seams of the OSB (under the tarp) with Wigluv which is waterproof but vapor open. No vapor barrier and the tarp is just temporary to protect the flat roof section from rain until the roofing is finished.

  10. jberks | | #10

    Hey GBA's,

    I'm enjoying this conversation because I'm getting ready to install the roof assembly on my new flat roof.

    We're talking about taping the seams of the deck regarding air barriers. I was wondering if anyone has any insight on applying an entire air SA membrane to the deck before the insulation in lieu of taping the seams? I've had some roofers tell me it overkill becasue the polyiso insulation would provide the air barrier? Thoughts?

    My roof assembly is based on Big Joe's article on over-roofing where he did this:

    To which the assembly would be:
    3/4" plywood deck (drying to inside)
    SA Air/Vapour Barrier membrane
    8" polyiso insulation (untaped seams)
    1/2" roofing substrate
    SBS 2-ply roof with UV reflective cap sheet.

  11. GBA Editor
    Martin Holladay | | #11

    Jamie B,
    Your approach will work just fine. If you want to save money, taping the seams of the plywood roof sheathing (to create an air barrier) with high quality tape would probably be cheaper than covering the sheathing with a self-adhered (peel-and-stick) membrane.

    For more information on this type of roof, see these two articles:

    Insulating Low-Slope Residential Roofs

    How to Install Rigid Foam On Top of Roof Sheathing

  12. Jon_Lawrence | | #12

    My low-slope roof is finally done! The assembly consisted of Advantech 3/4" for the roof deck, seams taped with Siga Wigluv, 2" sheets of polyiso, 4 inches at the roof edge, 12" at the start of the run, and then tapered polyiso on top to create a 1/4" slope - see attached diagram. On top of the polyiso, is DensDeck cover board and then a TPO membrane. The majority of the roof space will be covered with solar panels and the balance will be a green roof, hence the TPO vs EPDM. My original plan was to install dense pack cellulose between the rafters, but now I am leaning towards CCSF as I have been able to find installers in my area who use Lapolla's 4G product. So I am looking at a single lift of 5.5" of foam vs 12" of cellulose plus a membrane.

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