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Community and Q&A

Cathedral Roof Assembly Options

justin_blejer | Posted in GBA Pro Help on

Hello, and thank you in advance for you input. I am an architect working in NYS, Zone 6 on a new construction addition to a house. We are working on insulated roof assemblies (finished and unfinished) and I have 2 assembly options that I believe meet code and are sound consturction. Looking for your input to see if there are simpler ways still or if I am missing something critical that in could improve these systems. 

Code wise, I am looking at:
– Table R402.1.2, Option 1 for Zone 6
– R806.5 Unvented Enclosed Rafter Assemblies including table R806.5

I am specifying Zip R for the first time. The owners used it on their existing house (this is an addition) and like it, and it is commonly used here. 

Some of my thoughts are that I have allowed for drying in at least one direction, and using readily purchased materials. The unvented feels like a better assembly as it has continuous insulation, but maybe potentially more complicated and costly, requiring more parts. 

Thanks again,

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  1. Expert Member


    First off I'd like to say how comprehensible your drawings are. Very clear and easy to read.

    I'd also suggest looking at this article to understand the common options:

    As long as the roof shape is not interrupted by dormers or hips, I'd opt for the vented option. it's a lot easier and cheaper to build.

    A few minor comments on both your sections:
    - I don't see any advantage to including a "breather" under the metal roofing.
    - You are missing a rim-rafter on the 2"x8" framing.
    - Is 3/4" plywood roof sheathing necessary?
    - I'd consider re-working the vent detail to use a premade product, not insect-screen, which will be easily damaged.
    - A rain-screen is always beneficial, especially with small overhangs.

    1. justin_blejer | | #2

      Hi Malcolm,

      Thank you for the comments, and the compliment on the drawings. Can you shed a little light on some of your comments:
      - Why is the vented assembly cheaper?
      - I have always seen breather used to keep any moisture from corroding the underside of the metal roofing.
      - what do you recommend instead of 3/4" ply? I have always understood plywood to be better for attaching the roofing to.
      - Do you have any insect screen products you can recommend?
      - I agree with the rain screen, the owner was concerned it would be another cost, and our siding (Hardie board) dosnt require it and won't be painted.

      I have read through that article, thank you. I always find the details of a specific project are different, slightly or a lot, from the base concept details. But I have found those helpful.

      I suppose the vented option could be modified to become unvented as a Spray and Batt system as per option 5 in the article.

      Thank you again!

      1. Expert Member
        MALCOLM TAYLOR | | #3


        - The vented assembly is cheaper in materials, as it uses only one layer of sheathing, and no rigid insulation or long screws. It represents much less labour as it is basically the bottom half of the unvented assembly. It also has the advantage of being familiar to most builders, and can be built in inclement weather.
        - Metal roofing panels have the same undercoat on both sides. There is no reason to believe they will suffer moisture damage on the underside, or that there will be any moisture accumulation there with either of your assemblies. The profile of the panels already provide a path for some drying. Small mesh underlayments assume some mechanism for dissipating moisture and moving air in a gap the is too small to do either. They sometimes make sense on walls as a capillary break, but I'm not convinced they add anything on roofs.
        - 1/2" plywood provides a good substrate for metal roofing. 5/8" is a good upgrade, 3/4" is a heavy expensive overkill.
        - I would use a pre-made continuous soffit vent or get a perforated flashing made up. I've worked on too many buildings with insect and pest infiltration through torn insect screening. And once it's damaged it is very hard to replace.
        - The rain-screen was just a suggestion. It makes more sense in some climates than others. I'm a bit c0nfused by your comment that the Hardy-Board won't be painted?

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