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Community and Q&A

Feedback on unvented flat roof detail

mlogan | Posted in Green Building Techniques on

I’m looking at putting an unvented, flat (2%) roof on an office shed I’m building in my back yard. I’ve read a lot of the articles about low slope roofs on this site, but would still love it if someone took a quick look at my plan:

– Climate zone 3 (California, SF Bay Area.)
– Flat 2×8 rafters directly on top of wall plate, with sloped furring strips on top to create 1/4:12 slope.
– 1/2″ OSB decking
– 2″ Polyiso panels (fiberglass face) on top of decking.
– 60 mil EPDM membrane.
– Open cell spray foam insulation on underside of decking (between rafters).
– 1/2″ painted drywall ceiling below that.

I’ve read some people advising closed cell foam on the underside. I understand that open cell foam lets the roof dry to the inside. However, doesn’t this also carry the risk of the roof deck absorbing interior moisture?

My other question is whether I need to ensure that the ceiling interior (between the drywall and spray foam insulation) is vented to the interior of the building, or if painted drywall is permeable enough on its own.


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  1. user-2310254 | | #1


    If you are already planning to put two inches of foam on the roof, why not put more? You probably could find reclaimed foam in your area.

    Will you heat and cool this shed/office?

  2. mlogan | | #2

    Sure, I could do more, maybe I will. But my main question is really about my overall design for the roof and whether open cell foam is really the best option underneath.

  3. Jon_R | | #3

    Consider replacing the spray foam with fiberglass batts. With the polyiso, you won't have a condensation/sorption problem, so it's fine for the remaining insulation to be vapor and air permeable.

  4. onslow | | #4

    Mark, Second the batts idea, lots cheaper and possibly a safer choice. My feeling is that batts will allow more rapid redistribution of moisture imbalances. If you did have a chilly roof deck condition, then allowing any condensate to redistribute rapidly is a better idea. In any case, I imagine your local temperatures will not be driving a lot of intense condensation scenarios. Does the local weather bring you lots of moisture on little cat feet?

    Be sure to tape the ceiling well to stop free air access with the interior and paint to moderate the permeability of the drywall. Don't punch holes in the ceiling with can lights. If you must put electrical boxes in for track lighting, seal them well. If there are no paths for bulk air exchange between the ceiling cavity and the room or outdoors, then there should be little worry.

    Sealing the rafter bays at the exterior walls will be the most labor intensive and annoying bit. It is also where the most risk of failure resides. I elected to have closed cell foam sprayed to seal in all my rafter/wall intersects. Everything seems in order after four winters which can go to -15F. Of course I can't peek, so maybe faith is at play here.

    A few other notes. You might consider adding two or three more layers of 2x stock to the top plate on the roof rafter's high side, or just build that wall 3-4" taller. Lots easier to cut two infills on square framed endwalls than a bunch of extremely tedious shims for the tops. Don't forget to trim the uphill tips of the rafters to keep the fascia true.

    While I can't say anything about TPO application, I can observe that my PVC membrane roof is anchored over two layers of fiberglass with substantial screws and heat bonded seams. Field areas are not glued and the 50mph gust we get do create wind lift on those parts. The fiberglass is for fire rating which you haven't specifically addressed in your brief description.

    Only you will have to answer for code where you are, but I suspect compliance with wildfire rules may force you to other choices. Even if you are allowed to build sheds up to a certain size, once you add electricity and drywall the rules may change substantially. Also, hope your neighbors are good friends.

  5. mlogan | | #5

    Thanks for the replies so far.

    Roger: We do have moist winters. November through April can be cool (low 50s during the day) and moist for weeks at a time. Not a harsh climate, but you can definitely get rot problems on anything that isn't drying properly. Batt insulation does sound easier, although I seem to recall reading that it is important to minimize air space under the roof deck in scenarios like mine.

    Regarding slope for the roof, I would definitely prefer to to simply stack the high-side wall plate a bit higher, but the reason I'm doing a flat roof in the first place is that I have one wall at 45 degrees to the main walls. (Picture a rectangle with one corner lopped off). This means you couldn't easily create a uniform roof slope by stacking one plate higher. Whereas, low-angle rips on a table saw might be annoying, but the design "just works". I suppose one would normally do a hip roof in this situation, but aesthetically that wouldn't match my house, nor do I want to build a hip roof.

    (The reason for this is limited yard space and required separations/setbacks, plus a utility easement. I'm operating under some annoying constraints.)

    I don't anticipate many fire-rating issues. We aren't in a wildfire area. The only restriction I'm aware of is when building less than 3 feet from the property line, but I'm not doing that.

  6. onslow | | #6

    Mark, I admire your tenacity with a table saw. As long as it works and you're doing the work, then that's the answer.

    Poetic references aside, your high moisture environment will demand good sealing of the insulated bays between rafters. Keeping "new" moist air out of the bays is key to preventing excess moisture build up on colder parts of the structure. It may be that for part of the year your air conditioned walls will be the coolest part of the structure.

    If there is no moist air infiltrating from outside then all should be fine. Others have posted about interior drywall issues with leaky houses and high humidity air infiltration. In the southeast with long seasons of cooling and high humidity, drying to the outside or attic is not going to happen, especially if the leaky house is pulling more air in all the time.

    The size of the gap between the batts and the back of the drywall is most relevant if gaps and holes are allowing "new" moist air to roam more freely. Stopping the air flow is key. Air leaks are more important than than the perm ratings most times.

    The same is true for walls of course, so plan on good detailing there as well. You might also want to consider going with 1/2 plywood on the roof to provide an extra measure of fail safe against what should be very mild risk under 2" of poly iso.

    Best of luck.

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