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Community and Q&A

Venting a low-slope shed roof

mdb_az | Posted in PassivHaus on

Hey all,
We’re in final planning phases (we’re the GC) of a passive house in upstate NY climate zone 6.
The house has a shed roof with 1:12 pitch (I know not great for snow, etc)
The roof assembly is to be (from inside->out)
-1/2″ GWB
-1-1/2″ furring
-Intello smart membrane
-16″ TJI with dense-packed cellulose
-Solitex Mento WRB
-2 x 4″ strapping in plane of roof
-5/8″ CDX roof sheathing
-High-temp ice and water shield
-standing seam roof (using a specially high raised seam for the low slope)
*There will be low and high soffit vents.

My question is: will the roof effectively vent with such a low slope? Is there a better way of venting it then the proposed construction assembly (presuming a 16″ TJI rafter regardless)? Is it safer to do a compact roof?

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    User ...222,

    I'll leave others to comment on the robustness of your assembly and venting.

    My concern with very low-sloped metal roofs are the eave, ridge rake and any openings. Typically 1/12 (essentially flat) roofs use scuppers, parapets and membranes to ensure that the water can not wick back by capillary action or wind. Metal roofs use lapped flashing which relies on a slope to shed the water in the desired direction. While high seams may make the field fairly safe, the edges need a lot of attention - and I'm not familiar with any details that would make them sufficiently robust. Consulting an architect or roofer with experience in these types of installations would be my first suggestion.

  2. jackofalltrades777 | | #2

    I have a 2:12 roof with a high-standing seam from MBCI roofing called UltraDeck. It is engineered to be used down a 1/4:12 slope. Yes, that's correct, basically 1/4" slope. Depending on the length of the shed roof, a 1:12 roof slope is NOT flat.

    The main thing is to get the correct metal roof for your applications. Most standing seam roofs require 3:12 pitch for proper water shedding. One has to use a metal roof that is engineered to be able to go down to your 1:12 or even 1/4:12 pitch.

    I've had rain rates of 10 inches per hour and the roof had no issues. It was coming off the eaves in sheets, like a waterfall, but no issues or leaks. Metal roofs shed water pretty quickly, more quickly than a shingle roof would. Contact MBCI and inquire about UltraDeck. It has a 3.5" tall standing seam.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Peter,

      "Depending on the length of the shed roof, a 1:12 roof slope is NOT flat."
      The slope of a 1/12 roof is independent of it's length. 1/12 is a ratio of rise to run.

      Your modestly sloped 2/12 roof is twice as steep as a 1/12 roof.

  3. mdb_az | | #4

    Hey guys, I appreciate the discussion of metal roof details but that's not the crux of my question which was about venting

  4. Expert Member
    MALCOLM TAYLOR | | #5

    User ...222,

    Since no one else has chimed in...

    The mechanism that moves air through a ventilated roof is the stack effect. For that to operate you need a different in elevation from the eaves to the peak that you don't have with a 1/12 roof.

    In the absence of that there is some recent evidence that diffusion could be used instead. How you would do that with a shed roof I don't know. Again, hopefully someone with a different perspective will be better able to answer your question.

  5. Expert Member
    Peter Engle | | #6

    Diffusion doesn't work very well in Zone 6. Venting doesn't work very well with shallow pitched roofs. You will still get some venting due to cross winds, but not much from the stack effect. Are you on a site with strong daily breezes, and is the roof oriented to take any benefit from them? With membranes above and below the dense-packed cellulose, you're not going to get much vapor movement from the interior, so ventilation becomes far less important. You might be OK.

    I would be very concerned about ridge venting details with that low pitch. Lots of potential for wind to drive snow and/or water up into your ridge vents. Then you need a LOT of venting to dry things out.

  6. Jon_R | | #7

    Flat roof vents in the UK are required to be at least 50-60 mm. IMO, it would be wise to meet that spec, even though your roof has some moisture advantages over the typical UK flat roof.

  7. GBA Editor
    Martin Holladay | | #8

    User-6878222,
    Fist of all, can you tell us your name? (I'm Martin.)

    There is a clear answer to your question. For ventilation channels to be effective, you need a slope of at least 3/12 or steeper.

    Low-slope roofs are difficult to ventilate, but there are a few techniques that work. Basically, you need lots of height above the top of your insulation layer, plus a few vented "doghouses" (cupolas). Most people don't like the way these vented low-slope roofs look, so then end up going with an unvented assembly.

    All of this is explained in the following article: "Insulating Low-Slope Residential Roofs."

  8. Expert Member
    Michael Maines | | #9

    User-6878222, if your Intello is installed well your roof will not see much moisture, Mento will not slow vapor drying to the exterior, a 1 1/2" ventilation cavity is more than code requires, and CDX can stand up to a bit of wetting without falling apart, so although it's not best practice, you may not see any problems with your proposed assembly. That said, a steeper roof would perform better.

    As an aside, are you actually trying to meet Passive House targets? I would be surprised if you can get there with an R-60 roof, unless your building is large and everything else falls in your favor. Most of the Passive Houses I've been involved with, including one in Woodstock, NY with a roof assembly similar to yours, and others in zone 6, have needed R-80 to R-140 in the roof in order to meet the PH standard.

  9. mdb_az | | #10

    hey all:
    I appreciate the comments:
    Michael: I'm sorry I should've been more specific: we're trying to hit the "Low Energy Building" standard; to get within range of that we'll also be insulating that 1-1/2" service cavity with mineral wool. That gets us to R-65.2
    At this point the ship has sailed in terms of slope and building massing. Really I'm trying to optimize for the assembly at this point. We're trying to keep foam out of the building but I don't want to sacrifice the longer-term building durability on that altar.
    Ya'll are very helpful and I appreciate it very much!

  10. GBA Editor
    Martin Holladay | | #11

    User 6878222,
    I still don't know your name.

    If you want a low-slope roof without any spray foam or rigid foam, there are only two approaches that will work, as far as I know:

    1. You need to make sure that you have an air space of at least 6 inches between the top of your insulation layer and the underside of your roof sheathing, coupled with several vented cupolas in the center of your roof (along with perimeter air intakes) as described in my article, "Insulating Low-Slope Residential Roofs."

    or

    2. You need to create an unvented roof assembly by installing semi-rigid mineral wool insulation on the exterior side of the roof sheathing.

  11. jackofalltrades777 | | #12

    Malcolm,

    I need to clarify. I understand the 1:12 ratio of roofs. The point I was trying to make is that a 2:12 roof run of say 70 feet vs 3o feet will play into how quickly it can shed water. The engineers at companies like MBCI stated that on a 100 foot run of 2:12 vs a 30 feet run of 2:12 will have different factors in play. The longer run naturally has more volume of water being collected and collection area before it sheds it off the eave edge.

    So how long the actual panel run is plays a role in determining how much water and how quickly it can shed it off. Hope that makes sense but that's how it was explained to me by the engineer.

    The slope of a 1/12 roof is independent of it's length. 1/12 is a ratio of rise to run.

    Your modestly sloped 2/12 roof is twice as steep as a 1/12 roof.

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