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Community and Q&A

Low-slope “warm” roof build-up clarification

Quinn Sievewright | Posted in GBA Pro Help on

We are advancing on the planning stages of a new build project in climate 4C (near Vancouver, Canada). Thanks to the many articles and Q&As on this site I’ve got myself comfortable with most outstanding issues we are facing in developing our roof and wall build ups. This is a small building used as a holiday home, not fully occupied in Winter.
With that in mind any feedback from your good selves would be appreciated.
The roof is a mono pitch (shed style) front to back at just 1:12 with no “rafter” space to hold insulation
Build up:
– Exposed 3X8 Douglas fir beams
– Exposed 2×6 Douglas fir T&G boards
– Vapour barrier – BIGGEST QUESTION – Just want to confirm that what we need is a fully-adhered air and vapour barrier, such as Ice & Water (I believe this qualifies?) or another similar product? Any product suggestions?
– 1″ poly iso foam (to boost the R Value of the assembly) We may need to drop this however due to budget.
– 2 layers of 3″ EPS (offset seams etc). Total R value is only R37 ish but our code for flat roofs is only R26
– 1/2″ ply sheathing, long screws down into 2×8″ beams to hold together assembly.
– EPDM membrane (possibly torch on).

The house designer (not necessary a qualified architect or experienced builder) says we need Ice and Water for ice dam protection, I fail to see in what layer of the build up that would go in the “warm” unvented assembly we are proposing and what it would achieve when one has a fully-adhered type roof like an EPDM membrane.

This is still in the planning stages so also open to suggestions for a low-slope roof with exposed rafters if you see us going wrong with the above in mind.
Thanks in advance.

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  1. Steve Knapp CZ 3A Georgia | | #1

    This roof assembly sounds problematic. Typically you would have an air-tight drywall layer at the ceiling, and your T&G would be installed over this. Are you planning to use Ice & Water on the interior as the air barrier?

    You might want to review these articles:

    How to Insulate a Cathedral Ceiling (

    How to Install Rigid Foam on Top of Your Roof Sheathing (

    Also... R-49 is recommended for Zone 4. If reclaimed foam is available in your area, you can purchase it for about one-third the cost of new product.

    Maybe I am missing something since this structure is not supposed to be occupied year-round.

  2. Quinn Sievewright | | #2

    Hi Steve
    Yes I understand you'd typically seal at the underside of the rafters, unfortunately that won't be the case here. The open rafter architecture is beyond my control but is commonly done, albeit perhaps not the ideal method. As such I am trying to come up with the best solution.
    I have read the articles you've pointed to. We will certainly try to achieve a higher R Value if costs allow and your idea of sourcing some reclaimed foam is definitely something I'll investigate, but from an assembly that works point of view we want to be conservative with how much rigid foam we are assuming at this stage. Thanks for your thoughts.

  3. GBA Editor
    Martin Holladay | | #3

    I disagree with Steve Knapp. Installing Ice & Water Shield as an air barrier above a tongue-and-groove ceiling is a time-honored method (pioneered by PERSIST builders in the late 1960s). It's a standard approach, and you apparently understand the principles behind the need for an interior air barrier.

    The only worry that I've heard (especially among people who call themselves "chemically sensitive") is the worry that some brands of peel-and-stick may have an odor. Whether this is a problem or not is a subject of debate.

    If you worry about odor, you can always install a layer of plywood or OSB above the tongue-and-groove ceiling as your air barrier. Of course, the seams would need to be taped.

    If you decide to use plywood or OSB as your air barrier, you could skip the peel-and-stick.

    If you have chosen to install EPDM as your roofing layer, there is no reason whatsoever to also include Ice & Water Shield "for ice dam protection," as your designer asserts.

    If you are worried about ice dams, you need (a) an airtight assembly, and (b) enough R-value. Your plan to install R-37 of rigid foam is OK (if legal), but R-49 would be better.

    -- Martin Holladay

  4. Steve Knapp CZ 3A Georgia | | #4

    Martin always gives excellent advice. If it were my house, I actually would want better separation between the conditioned space and the chemicals in the Ice & Water. (So plywood or OSB as the interior sheathing layer with taped seams.) But to be honest, I was more focused on all the grooves in the T&G and the potential for air leakage.

    Still, I am not clear on how you are reaching R-37 with 6 or 7 inches of rigid foam. I guess you could get close with Neopor, which is rated R-5 per inch, but it may not be the most budget-friendly option.

  5. Expert Member
    Malcolm Taylor | | #5

    You are bumping up against the BC building code requirements, which don't always align with what you would do from a building science perspective.

    You do need an interior vapour barrier, although that could be a number of materials and doesn't have to be "fully adhered", although that will help during construction. Depending on the permeability of the foam you choose it could be the foam, it could be poly. or it could be an underlayment like Grace Tri-flex. I'd be inclined to use the I&WS.

  6. Expert Member
    Dana Dorsett | | #6

    Both polyiso and EPS run about the same $/R, and there is no advantage to the additional layering. Going with two layers of 3"-3.5" roofing polyiso is probably cheaper than your 1" polyiso + 2x3" EPS solution.

    If you used reclaimed roofing foam (both 3" and 3.5" are common standard thicknesses) it's greener too. Rigid foam of all types show up regularly with reclaimers, surplus and salvage materials dealers, often at a small fraction of virgin stock goods at the local distributors, which can extend the budget quite a bit. eg: I don't know what business these folks are in, but they seem to have a bunch of attractively priced surplus 4" XPS , even if the 2' x 4' sheet sizing is on the small side (2' x 8' are pretty common, as are 4' x 8'):

  7. Expert Member
    Malcolm Taylor | | #7

    The full quote was: "If you have chosen to install EPDM as your roofing layer, there is no reason whatsoever to also include Ice & Water Shield "for ice dam protection,"

    There wouldn't be any sandwiching. Both membranes would be on the exterior of the roof sheathing.

  8. Quinn Sievewright | | #8

    Thank you all for your detailed advice. Dana, I've tried to contact the individual in the Craigslist and hope they get back to me as it seems two layers of that would simplify things and certainly save some money. Thanks for the link and info!
    Malcolm, I see your point regarding the interior vapour barrier. I'm not totally clear from the code what is considered "interior". If it is above the roof sheathing but on the "warm" side of the insulation, hopefully they'll be fine with it.
    I may post again with the floor assembly once we get there! Thanks again.

  9. Jon R | | #9

    "...there is no reason whatsoever to also include Ice & Water Shield... "

    And a good reason not too (edit: in the position you list it - sandwiching plywood between two impermeable layers is problematic). Or, if someone is proposing under the EPDM, then it is wasted money.

  10. Tim R | | #10

    A T&G ceiling may not provide an adequate diaphragm for seismic forces on the west coast. This would lead to the solution of placing sheathing over the T&G for the roof diaphragm and thus the air barrier. You didn't indicate the spacing of the 3x8 beams but you may need to use staples on the roof diaphragm if there are not framing members at the correct spacing for the panel edges or increase the 2x6 to 3x6 or 3x8 to be able to create an adequate roof diaphragm with staples. If a fire rated roof assembly is needed then you may need a layer of DENSDECK below the EPDM roofing, the DENSDECK can replace the top layer of plywood over the foam.

  11. Expert Member
    Malcolm Taylor | | #11


    The code asks that the vapour barrier be "close enough to the interior to avoid condensation at design conditions". In other words, if you push it towards the outside you need to be able to show it won't be a problem. Your stack-up, with the insulation above, would be fine.

    Tim brings up a good point - and one that I missed: In the coastal seismic zones, roofs have to be sheathed with panel goods like plywood or OSB, and fastened according to a schedule of nailing or screws. It's unlikely that having the plywood separated from the structure by foam would meet this requirement.

  12. Quinn Sievewright | | #12

    Tim and Malcolm - thanks for this. The beams are 4' centres at the moment. We are at the design stage so I will have the engineer check on the roof build up from a seismic requirement point of view and confirming spacing for the loads. We can then decide if we replace the membrane vapour barrier with a structural panel with taped seams and either adhere the EPDM directly to the rigid insulation (problematic in my experience) or to something like Densdeck or another panel. Firestone also do a Fire Rated EPDM.

  13. Expert Member
    Malcolm Taylor | | #13

    The biggest problem with roofs using t&g as their ceilings is air-sealing if the boards extend out to form the roof overhangs. I don't have a good solution for that, but it is worth thinking about how you will deal with the wall/ceiling intersection.
    Good luck with your build.

  14. Quinn Sievewright | | #14

    Hi Malcolm - indeed I've been thinking about that same issue. On the sides of the building the t&g will sit directly on top and parallel to the 4x8s, so I was thinking of applying a layer of sealant under the t&g as we lay that board, perhaps a bead of sealant along the groove of the board as well before putting the tongue in at that location too.

    Not sure if it needs to expensive CONTEGA HF type stuff or if there is another option (another question I have!).

    On the high and low sides of the building the t&g is perpendicular to the 4X8s so was hoping to seal to the blocking placed between the rafters above the wall plate and to the top of the 4x8 beams. Also, may need to us a quality air sealing tape at all the 4x8 rafter penetrations, but at least this location will be covered by drywall so the air sealing can be "hidden". I understand these 'penetrations' of the building envelope are far from ideal but there is nothing I can do about that!

  15. Tim R | | #15

    If you look at the Firestone site for the class A fire rated EPDM Roof assembly it requires a layer of DensDeck just below the roof membrane. That can be screwed down then the EPDM glued to that.

  16. Expert Member
    Malcolm Taylor | | #16

    That's probably the best solution. Because there will be a bit of movement as things dry (and seasonally), something like acoustical sealant might be the best bet.

  17. Quinn Sievewright | | #17

    Hi guys again - Had a meeting with the designer and they would prefer to add some 2x8s on flat across the top of the insulation, then ply then EPDM on top of that in order to introduce some ventilation. There would then be vents in the exterior soffit front and rear. Is there any merit in that approach in such a low pitch roof assembly? Thanks again

  18. Expert Member
    Malcolm Taylor | | #18

    A roof with that slope will experience negligible benefit from ventilation, and why 2"x8"s? Neither makes much sense to me.

  19. GBA Editor
    Martin Holladay | | #19

    If you want a vented roof assembly, you should follow the recommendations in this article: Insulating Low-Slope Residential Roofs. (Here's the short version: 2x8s on the flat won't work.)

    The approach we have been talking about up until now is an unvented approach. It's routine for commercial roofs. If your designer and contractor have no familiarity with commercial work, that might explain their confusion on this issue.

    -- Martin Holladay

  20. Quinn Sievewright | | #20

    Thanks Martin - I suspected as much having read your articles however the "I've been doing this for years" kind of attitude I'm facing had me wanting to confirm from more knowledgeable experts in this specific area such as yourselves. I'm not trying to undervalue the contractor, he seems very knowledgeable generally, but they can't be experts in everything and are hopefully still willing to learn something new. With that in mind I'd like to post on the floor assembly as well to ensure I'm not being lead down the wrong path so to speak. Again, much appreciated.

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