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Residential flat roof design

user-735982 | Posted in General Questions on


I’m an architect looking for advice as to how to detail a wood frame, flat roof/parapet wall assembly. I looked for one on this website and there isn’t one. By flat I mean, one one that is sloped to drain and either is topped with a built up roof membrane or a single ply membrane. This is located in a mixed climate (Northern California).

I’m tending toward having an unvented assembly as the configuration doesn’t lend itself to a simple venting solution. How should I insulate this – fully insulate with fiberglass batts, cellulose, spray foam?

For water protection, what are people’s thoughts on a built up roof system versus a single ply membrane. Any suggestions as to a product I should check out?

Thanks in advance,


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  1. Chris | | #1

    We have used rigid on top of wood sheathing and then Sarnafil or EPDM over the rigid. Typically we have used 1-2" of rigid at the shallowest point of the roof and then slope up from that at a 1/4" per ft.

    With the rigid on the roof the framing can be insulated easily with spray foam or blown-in cellulose. You should take a look at the Sarnafil website. They have a lot of details for flashing at the parapet, roof drains, scuppers, etc.

  2. Riversong | | #2

    You might consider a living roof to reduce the albedo, offer evaporative cooling, and control rain runoff.

  3. Christopher Briley | | #3

    Start thinking commercial construction. You're shouldn't need to reinvent the wheel here. Go to a membrane roof manufacurer's website like If you top the insulation with a high density fiber board then you can really pile on the rigid insulation and get the r-value you want. That, and when it leaks it'll puff up arounfd the leak so you know right where it is :)

    Living roofs rock, but are pricey. zero heat gain in the summer, wonderful stormwater control, a pinch of extra r-value, kinder to the biosphere. I'm about to install a high pitch (3:12) living roof with xeroflora as the mfr. wish me luck


    We did a flat roof parapet design in a home that is profiled on this site.

    It's basically just a sloped floor with 3/4 T&G deck with a standard EPDM roof over and 8" of spray foam under. We wanted to walk on it so we covered it w/ Ameridrain and then two layers of Hardi backer with flashing between the layers to keep water and cigarette butts off the EPDM and then tile for a walk surface.

    The owners have a gas grill and raised bed planters up there. the space between the EPDM and the Ameridrain is drained into a subdrain similar to a shower subdrain and the tile has a outlets into the gutter system to the rain garden as well as a hurricane overflow outlet.

    The house is on the free GHBT home tour on May 15th (day before the NAHB National Green Building Conference in Raleigh and May 22nd. Come talk a look.

    We've also protected these type roofs with dog grooming mats for lighter use and even built a Koi pond on one. Brick or tile over Ameridrain is my favorite solution since they inevitably get a lot of traffic. For planting them I like galvanized steel pans for planters. 28 gauge 3' x 8' sheets are cheep and fold nicely into 2'x3' planters with 6" sides.

  5. Armando Cobo | | #5

    A big percentage of homes we build in the Southwest (NM & AZ) have “flat” roofs. They are designed with open web trusses with ¼” slope. Normally we use 7/16 OSB roof sheathing and bituminous roof membrane (torch applied), vents in the parapet walls and netted blown insulation. For better performance, you may want to try non-ventilated attic with 6”-10” open cell foam below the 7/16 OSB roof decking and 1”-2” rigid insulation on top of the roof sheathing to prevent thermal bridging. You can then apply the bituminous roof on top.
    If you are going to use the roof as a view-out deck or any other activity, make sure the trusses are designed for that load. Install ¾” OSB and (Trex like) decking material to protect the roofing membrane.
    If you email me at [email protected], I’ll send you a detail or two.

  6. MKT Trading | | #6

    Does anyone have a comment on IRMA roof for this application?
    Seen a lot in Vancouver, BC, but not in USA.

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