Detailing a Flat Roof with Parapet Walls
I am considering a flat roof with parapet walls on 3 sides and 3′ overhang on low side for the 32×40 main roof of a new home. California Climate Zone 16, dry mountains 7000ft but heavy snows.
I was thinking of 0.5:12 slope with these layers, top to bottom:
1) Gravel, rock, or unsecured porcelain tiles as ballast
2) R6 Rockwool
3) R15 EPS
4) R13 Polyiso
5) Grace Ice & Water
6) OSB sheathing
7) 2×8 rafters 12″ OC (max spans between supports only 9′)
8) R22 cellulose
9) smart vapor retarder
10) sheetrock ceiling
This seems like what Joe L. describes as a “leaned over” Perfect Wall to make a perfect roof, with the exception of (8&9). But … every other article I’ve read here places the roofing membrane, usually EPDM or TPO, above the foam insulation and the foam directly above sheathing taped as an air barrier only.
Is the 10 layered approach above not a good method ? Seems like it would drain and dry fine, and at a total <$10/sf for an R56 roof assembly, not overly expensive.
I like the ballast idea because no penetrations of the Grace Ice & Water for fasteners, but is it enough to hold the insulation layers in winds ? All plumbing vents, HRV, range makeup air, etc. would go through walls, as I am trying to avoid roof penetrations completely.
I think setting 12×12 light colored porcelain tiles out as ballast that weigh 3+lb/sf would work great. Water and frost proof, easy to pick up and put back if a leak needs patching, generally always on sale somewhere for <$1/sf, create an energy saving reflective “white roof”, DIY friendy. Any reason why they would not make a good ballast ?
Is Grace Ice & Water not good enough for a membrane even though it is protected under all these layers ? 60-90mil EPDM seems like overkill for something never exposed to sun, wind, hail, etc.
Should I have drainage mat of some kind between the polyiso and the Grace (4) and (5) ? Would a dimple mat of some kind be preferable to Tyvek Drain for a horizontal application like this ? Seems like even a thin layer of gravel, or landscape netting would work if it wouldn’t damage the Grace.
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Can you link to Joe L's recommendations? Like you I have never seen a flat roof where the waterproof layer wasn't either exposed or directly under the ballast.
Sure, here is the article about the "Perfect Wall" and how tilting it horizontal makes for the perfect roof and perfect foundation: https://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall
Notice that in Figure 2 there is a "filter fabric" under the ballast but all the control layers, from insulation down to the membranes to control air, water, and vapor are shown as the dark triple line between insulation and roof structure. My plan uses R6 ComfortBoard as the "filter fabric" layer to add insulation value and protect the foam layers from UV.
It really is the same as you would do for exterior wall insulation, with ballast in place of wall cladding. Having roof insulation under the water control layer like EPDM is definitely not the same.
Thanks Kirk. It's well beyond my expertise to comment usefully.
In California and building in a wildland interface zone - you need to make sure to specify a roof with a class A rating.
Thanks for the reminder. It appears that gravel and rock ballasted roofs are Class A rated. Since porcelain tile is fireproof and so is the Rockwool under it, there shouldn't be a problem. Do you know of some reason there would ?
I have gone thru some plan checks where they want the UL roof assembly number. You should check with your Permitting agency to see if they will approve what you are proposing.