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Community and Q&A

Venting a Shed Roof

russell1313 | Posted in Energy Efficiency and Durability on

I’ve been around and around on trying to determine the correct design for my new construction roof. In lower zone 4 – looking to do about r40 or better – the shed roof is basically TJI joists – 16″ or better, on 2x walls . Seal the assembly airtight – complete structure, I will be using standing seam roofing. 4/12 pitch. Initially I was going to fill the cavity with insulation, cover with Zip roof sheathing,  and install the metal.  Do I need to vent it  or not – strapping on Zip then roofing, or OSB with vapor perm layer and then strap, or should I vent under the Zip.   I have read everything here – and I’m still confused – what am I missing. Thanks all.

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Replies

  1. walta100 | | #1

    In my opinion the smart move is a flat ceiling covered with cheap fluffy insulation and a vented attic.

    I understand this may not work with your architectural goals but this plan and simple design will perform better and or cost less to build.

    We see the question every few days "help me fix my sloped ceiling" because there is never enough room for ventilation or acquit insulation and of course they must have a hundred pot lights in the sloped ceiling.

    Walta

  2. Jon_R | | #2

    Code is a good starting point. Yours could vary, but I'd also use this one. R806 is only a page or two:

    https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction#IRC2018_Pt03_Ch08_SecR806

  3. Expert Member
    Michael Maines | | #3

    There are many different ways to design a roof system like yours; the one that's best for you depends on your building code, but also your tolerance for risk, your builders' skills and experience, your budget and other elements.

    Personally I'd vent the roof. In zone 4 I'd be comfortable doing it above the Zip sheathing, with cross-hatched furring to provide ventilation and nailing. I'd add a waterproof underlayment over the Zip; Zip is usually watertight when installed correctly but the air gap under the roofing will create condensation in certain conditions and I would prefer to rely on something other than Zip alone to handle regular wetting for the life of the roofing. For that assembly you would also need metal roofing that can be installed on purlins, which might be a heavier gauge than you're planning to use.

    My second choice would be a sarking membrane system, with a good WRB meant for roofs stretched across the rafters, then 1 1/2" furring placed along each rafter, then the Zip sheathing installed normally. That's probably the best from a performance point of view and it's not as hard to do as some think, but it's not standard in the US and it can be awkward so your builder might balk.

    Third choice would be to vent the interior. The I-joist top flange is a handy depth: you can attach a membrane or rigid product against the underside of the top flange to create a vent space. I like 1/4" plywood for this situation, as it's vapor permeable, relatively low-carbon and easy to install. But I'm not sure where pricing is right now. Others use rigid foam, which is easy but unnecessarily high in embodied carbon, for new material. Or you could use a membrane but proper installation could be challenging.

    Last choice would be to make it a hot roof, aka unvented roof, which would require spray foam. With all of the other options I don't see why you would choose that one.

  4. russell1313 | | #4

    I should have mentioned I have no building code limits, just Elec and septic -I'm out in the county. However, my goal is build a higher performance home than most build in this area. I am also the builder and chief bottle washer. MM's number 2 suggestion seems interesting, and 3 is very doable, thanks for the replies. Funny I just found this in GBA very close to my roof layout.
    https://i.pinimg.com/600x315/09/90/2b/09902b9c3d7da69ebdf3ee59fb668271.jpg

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