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Venting a low slope un-vaulted roof

mdb_az | Posted in General Questions on

As always I try to avoid the use of spray foam but a client we are designing a new home for is adamant on wanting low slope roofs (for aesthetics) and I cannot dissuade them 🙂 The roof slopes are 2:12 and 1:12 respectively. I know that some consider 3:12 the minimum allowable slope for a vented roof assembly but I was curious if that applies just to vaulted assemblies or unvaulted assemblies as well? In this case I could design the roof unvaulted and insulate the flat ceiling portion leaving a generous amount of air circulation space above the insulation (say 6-8″ of free airspace overtop of the insulation). Would that be sufficient to overcome the dimished stack effect?

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Replies

  1. matthew25 | | #1

    Is exterior insulation not an option?

  2. Malcolm_Taylor | | #2

    mdb_az,

    The problem with low sloped roofs is that a slight reduction in slope on them makes a much larger diminution in function that say going from 6/12 to 7/12. You can successfully vent the roofs, but the chances of problems are a lot higher with the 1/12 one.

    With a large airspace, diligent warm side air-sealing, and lots of ventilation also located on the rakes, they should work.

    A bigger question is whether it ever makes sense to consciously design risk into new building assemblies. I have a short list of things I just won't do in our climate. Being in the PNW they include no overhangs, and low-sloped or flat roofs with living space under them.

    As Matthew says: Have you considered an un-vented assembly with foam above the sheathing? Assembly #3 in this link. https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

  3. Expert Member
    Michael Maines | | #3

    This article covers exactly that topic: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.

    Short version: with at least 6" clear space, a lot of attention on air-sealing, and with one or more central mini-cupolas, it can work.

  4. mdb_az | | #4

    thanks for the thoughtful reply Malcom! yes if we go vented i would definitely do warm side air sealing and lots of ventilation. The current design does feature big overhangs which is a welcome change from the trend of minimal/no overhangs.

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