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Best detail for EPDM on SIP roof? Second Try

ZEQfbYvNAN | Posted in GBA Pro Help on

I’m surprised I didn’t get no even one taker on my question. Perhaps it’s because I posted it under the wrong Q&A Category (Energy Efficiency and Durability), so I’ll try again here.

I will be self-contracting my own residence. SIP walls and 1/4:12 slope roof, EPDM roofing material.

Unfortunately, when I have to explain to local builder/suppliers what a SIP is, any answer I might get locally does not seem reliable to me.

So I have searched the web to find how the best approach, is but no luck yet.

From my understanding, SIPs need to dry to the interior (gypsum board, no poly) and the exterior. The latter is the part I’m having trouble with. EPDM will not allow the outside OSB dry, right? Do I need to add a second roof deck over purlins, and apply the membrane on top? Any other ways to do this?

House to be built in Walla Walla, Wash. 99362

Thanks for your time,

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  1. Riversong | | #1

    If, in fact, your roof structure is SIPs (STRUCTURAL insulated panels), then I would not place a non-breatheable membrane directly on the highly moisture-vulnerable OSB. Build a ventilation channel and apply a secondary roof deck (preferably plywood). Since both the OSB and the foam core form the composite structural element, you cannot afford to risk deterioration of the outer OSB skin. OSB needs to be able to dry if it's going to remain durable.

  2. ZEQfbYvNAN | | #2

    Thanks Robert. That's exactly what I was concerned about, trapping moisture on the SIP outside OSB layer.

    It sounds like the roof will be best built with a secondary roof deck. Since it rains less than 10" in Walla Walla, do you think 1/2" (or less) purlins will provide enough ventilation?

    And finally, just in case any moisture makes it underneath the secondary roof deck, should I use any protection, like a Tyvek product, on top of the OSB, underneath the purlins?

  3. Riversong | | #3

    A breatheable weather barrier membrane on the SIPS would offer good secondary protection, since there is not enough pitch to drain liquid water. Any space above the OSB would help. Normally, at least 1½" of space is necessary for good stack effect ventilation. But with your nearly flat roof, you're not going to get any airflow unless the wind is blowing. Make sure you leave the vent channel open at the lower and upper roof and screened for insects.

  4. ZEQfbYvNAN | | #4

    Just found a technical bulletin from the SIP manufacturer regarding low slope roofs and fully adhered membranes:

    "Therefore, R-Control requires that a Dens-Deck (1/4” thickness
    or greater) or wood fiber board (1/2” or greater), or
    similar coverboard, in the type and style approved by the
    Low Slope Roofing System Manufacturer, be attached on
    top of the R-Control SIP roof deck prior to the installation
    of fully adhered low slope roof covering systems. Mechanical
    attachment of the coverboard shall be installed in accordance
    with the Low Slope Roofing Manufacturer’s recommendations
    for application to a 7/16” OSB deck."

    Would these DensDeck or wood fiber board work without the air space we were talking before?

    It seems to me any moisture wouldn't have a way to dry out ...

    I like the idea of a secondary roof deck, but the option of using a simple coverboard sounds simple and quick.

    Any thoughts?

  5. Armando Cobo | | #5

    In the SW states, we have been building “flat” roofs for hundreds of years mainly for the cost savings and the fact that we have very low rain amounts helps. ¼” or ½” slope is sufficient to get proper drainage. EPDM roofing systems are very good if installed correctly; they can last upwards of 30+ years with proper maintenance. You also need to build crickets that direct the water to the canales, parapet drains or siphons and proper flashing should be used. I would make sure that your SIPs have an R38+ or higher and control you interior humidity and ventilation. As with any whole house system, design, details and execution is the key on whether you have callbacks or not. You shouldn’t need to waste money on a secondary roof deck. IMH&EO, E for experience.

  6. Riversong | | #6

    I have no issue with flat roofs. I have a legitimate concern about OSB covered with an impermeable membrane. Thats a recipe for failure. Studies have supported that concern.

    A fibrous board separating the OSB from the membrane would be an improvement, but not nearly as durable as a vent cavity. Even unvented cavities have proven superior to a sealed surface.

  7. jwyman | | #7

    My office is Western Massachusetts has a partial flat SIP roof that was padded out with 2x sloped sleepers, fiberboard and a single-ply membrane. The low roof helps with daylighting another space. I agree with Robert and the installer who would not install the membrane over OSB.

  8. Robert Car | | #8

    Thanks for sharing your experience, Armando. The problem I have is the SIP manufacturer does not want EPDM directly on the OSB. If I have to put a secondary roof deck, I might as well have ventilation under it. What Robert and Jon suggest seems to be the best solution, considering the little rain we get here and snow in winter (lot's of houses with ice dam problems in this area). The ventilation in between will avoid the ice dams—am I correct?

    One more question: what specific wrap should I put right on the SIP, underneath the purlins? My reading on the DuPont site shows DrainWrap like it might do the work. However, it's always shown on walls, not roofs.

    Would DrainWrap work, or is there another product I'm not seeing?

    Am I the only one building a roof like this?

    Thanks again for everybody's time.

  9. Robert Car | | #9

    Jon: beautiful office building you have! Thanks for sharing.

  10. Riversong | | #10


    Ice dams are an indication of excessive heat flux through the roof, so the primary method of prevention has to be to increase the roof insulation and decrease heat loss. As a secondary measure, roof ventilation can help keep the roof surface cold by allowing convective air flow. But that requires either a minimum of 3' of head and a large enough vent channel (usually a minimum of 1½". A flat roof doesn't allow for stack effect flow, only wind-forced flow. But, never-the-less, isolating the roof deck from the thermal envelope will help reduce roof temperature and ice dam probability.

    I would think that any non-perforated WRB would be fine, including half-lapped roofing felt.

  11. Riversong | | #11


    Jon, that office certainly is a beautiful structure. But I wonder about the cedar-shingled slant walls. I worked on an architect-designed renovation in Rochester VT which included 5° slanted cedar-shingled walls under roof overhangs (much like yours) and the lower courses of cedar took a real beating from roof runoff.

    Is this a case where aesthetics has trumped good building practice and durability?

  12. Vhoof | | #13

    Robert -

    Your air/vapor barrier needs to be on the "warm-in-winter" side of the panel. Technically the OSB counts as a vapor barrier; the problem is the joints (thats why we mandate SIP tape).
    As for the roof deck, a vapor permeable weather barrier is preferred if the panels prior to installation get any moisture at all. If the SIP's are installed dry & correctly and your building is vented appropriately, you should be able choose your weather barrier (permeable or non) and install your EPDM.

  13. Robert Car | | #14

    Vhoof, thanks for your input as well.

    I'm planning to use SIP tape on ceiling and wall interior panels.

    On the exterior roof panel though, I'm assuming the detail you suggest is on also agree on a secondary roof deck, correct? Or do you have a different approach? Are you a SIP manufacturer?

  14. Riversong | | #15


    Actually only the vapor barrier must be on the warm-in-winter side of the thermal envelope. The air barrier can be anywhere within the thermal envelope (inside, outside or midline) as long as it is continuous and contiguous with the thermal barrier.

    OSB does NOT qualify as a vapor barrier. It has a dry cup perm of about 0.75 and a wet cup perm of about 2, so it is a class II vapor retarder (semi-impermeable). Yes, a major problem with SIPS, particularly on a roof, is moisture-laden air leakage at the joints. But OSB, in itself, is highly vulnerable to moisture and must be protected thoroughly.

    A 10-year computer evaluation of a variety of roofing systems in both hot and cold climates by the Florida Solar Energy Center demonstrated that an unvented roof deck covered with an impermeable membrane - particularly when a relatively impermeable insulation is in contact with it underneath - is extremely vulnerable to degradation in the event of a roof leak (which happens at some point in the life of every house).

    Best practice is:
    1) avoid OSB whenever possible, particularly in a roof assembly
    2) use vapor-permeable insulation below and roofing above OSB roof sheathing
    3) vent the roof to allow drying potential in the event of either vapor accumulation or a bulk leak

  15. jwyman | | #16

    The roof overhangs are 3'-6" and the walls slant at 2'-0". So the overhang still protects most of the sloped walls. On occasion, rain will soak the outside corners, but that can be expected on a plumb wall too. I appreciate the comment!

  16. Anonymous | | #17

    Question: I am building a home with a shed roof that spans 26 ft with a 2ft difference in thd front wall. 2ft over 26 ft for pitch. Front south wall is 12 ft the rear is 10 ft. Any suggestions for the best type of rubber on SIP roof. Anyone recommend spay rubber roof application?

    Thanks for any help!

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