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Help with a 1:12 flat roof for a Brooklyn rowhouse

Armen Simon | Posted in Energy Efficiency and Durability on

Hello. I wanted to get some opinions on the best roofing options for the flat roof of my 1860’s Brooklyn wood frame row house. As part of  gut reno,  I’m going to rip all of the existing roofing off down to the beams so really all options are open. The architectural plans call for a 1:12 slope and there is also a wood deck tile area so there will be some foot traffic. I am leaning toward the Gaff Liberty Low Slope system as the 3 ply method seems to me to be a more durable option than a single sheet of EPDM rubber. I also want to make sure I have 2″ of exterior insulation on the roof as well. Any opinions on how the smartest wall assembly would be much appreciated. Does plywood sheathing even make sense if there are products like Densdeck out there? Thanks, Armen

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  1. Expert Member
    Dana Dorsett | | #1

    It takes far more than 2" of exterior insulation to hit IRC code minimum for zone 4A even on a U-factor basis. That isn't enough to even provide dew point control for a code-minimum stackup with some of the insulation under the roof deck.

    It takes 6" of reclaimed (or new) roofing polyiso tomeet code min on a U-factor basis.

    It takes 3" of roofing polyiso plus R28- R30 rock wool snugged up to the underside of the roof deck to get there on an R-value basis.

    The wall sheathing requirements vary depending on the framing design. If there is cross bracing built into the framing it's fine to use less-structural sheathing. The depth of the studs and the type of sheathing make a difference on what makes sense for wall insulation.

    1. Armen Simon | | #2

      Dana thanks. I will have rockwool comfort batt in the ceiling which is how I’ll mostly get to the R49 I need for code compliance. What roof assembly would you reccomend for plywood sheathing on up... ? Thanks

      1. Expert Member
        Dana Dorsett | | #3

        With R49 total at least R15 (and preferably R20) of that total needs to be on the exterior of the roof deck. If at the bare-minimum R15 it is important to have an excellent air barrier on the under side of the rock wool, with a vapor retardency between 0.5 - 3.0 perms, and the top side of the batts has to be in contact with the roof deck. At R20+ above the roof deck the tightness and vapor retardency of the interior side air barrier less critical. Most 2lb roofing polyiso is labeled R5.7/inch, so it takes a minimum of 3" to meet the IRC prescriptive minimums for dew point control on R30 rock wool, but 4" would be a lot safer/better.

        Using reclaimed goods can make that affordable- it's often cheaper than batts. One way to find local foam reclaimers is to run this search every week or so:

      2. GBA Editor
        Martin Holladay | | #4

        Dana is right. For more information on this issue, see these two articles:

        "Combining Exterior Rigid Foam With Fluffy Insulation"

        "Insulating Low-Slope Residential Roofs"

  2. Armen Simon | | #5

    Thank you guys. Do any of your recommendations change given that this 16'x43' roof will have a 10x10' Ipe wood deck area? I've attached my roof plan for your review/comment.
    Here's my revised assembly for the R49 roof from bottom up.. please let me know thoughts. Regards, Armen

    60 Mill EPDM membrane - self adhered
    4"-5" XPS (some kind of high compressive strength)
    3/4" plywood sheathing, taped seams
    Rockwool R23 Batt Insulation
    ProClima Intello

  3. Expert Member
    Dana Dorsett | | #6

    XPS is one of the least-green insulation materials in common use, due to the HFC blowing agents (extremely powerful greenhouse gases). It also loses performance over time as the HFCs diffuse out. While it's warranteed to be at least 90% of it's labeled R value (R4.5/inch, not the labeled R5/inch), over several decades it's going to slowly drop to about R4.2/inch, the same as EPS of similar density.

    Polyiso is blown with hydrocarbons with very low environmental impact, has a higher R/inch, and uses a less damaging polymer.

    Even at the labeled R5/inch, R23 rock wool + 5" of XPS does not hit the code-min R49, (though it would almost certainly be compliant on a U-factor basis.)

    But 5" of 2lb density roofing polyiso would be labeled R5.7/inch (R28.5 @5") and perform at least to R5.5inch (R27.5) in your stackup, and with R23 rock wool under the roof deck would beat the R49 code minimum.

    You don't need a high compressive strength foam to support a deck- it just needs a design. While 2lb roofing polyiso is nominally "walkable" under a membrane roof, to support a deck would require some sleepers to distribute the weight and eliminate potential high compression zones. Even 3lbs XPS would require designing the deck supports. The timber for the supports is far cheaper than using crazy-high-density foam over the entire deck.

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