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Cathedral ceiling and roofing design for great room

Mr_Dude | Posted in General Questions on

I wish to submit a design for cathedral ceiling and roofing for review.  I did submit a prior question regarding the venting of eaves with this design back on 26-Jul, however, never took the opportunity to submit the complete design.  Following are the details:


1.      Location:  Suburb north of Chicago, therefore Zone 5 . . . we are ~ 15 miles south of the Wisconsin border which is Zone 6.

2.      Drawing:  Attached is a cross-section of our Great Room design.

3.      Great Room Size:  Span of 40′ wide (as viewed in drawing) and a depth (into the page) of 36′.  The room is symmetrical along the 36′ axis.

4.      Roof Pitch:  9:12.

5.      Construction:  Supporting the roof deck are two end gables and 3 intermediate structural timber scissors trusses, all spaced at ~ 9′ on center.  Atop each truss will be a 5/8″ plywood strip (see Ceiling Material below).  Atop these strips, 2 x 12 Douglas Fir purlins will run perpendicular to the end gables and intermediate trusses.  7/16″ thick OSB sheathing will be installed atop the purlins (see Sheathing Material below).

6.      Insulation:  7-1/2” of closed cell foam . . . while the insulation area has an R-value of 48.8, the effective R-value of the system is 34.5 (considering thermal bridging).  The 2 x 12 purlins give us 11-1/4″ of depth for insulation, low profile insulated cans [with LED lamping], and miscellaneous electrical boxes for pendants and fans.

7.      Ceiling Material:  1/2″ gypsum board will be installed to the underside of the purlins, with each drywall end able to be ‘tucked’ into the 5/8″ space above each truss . . . this will eliminate drywall taping against the trusses and enable efficient painting and ‘clean’ lines.  Importantly, there will be no seal between the living space and deck cavity below the insulation.

8.      Sheathing Material:  To mitigate the effects of rain between the time of roof sheathing and roofing installation, we are planning to use the Huber ZIP System sheathing and tape to address this potential wet time period.

9.      Roofing Build-up:

·         Grace Ice and Water Shield over entire roof surface

·         Atlas StormMaster Class 4 asphalt shingles

10.  Other Considerations:

·         Environmental Implications of Closed-Cell Foam – We are aware of the environmental impact of the HFC-245fa blowing agent and its GWP of 950 at 100 years.  We are reviewing alternatives, but may not be able to change our design.

·         Consideration of SIP Panels – We did a thorough review of this option, however, the need to accurately spot locations for electricals ahead of factory fabrication was deemed too challenging.

·         We are considering the addition of 15# felt on top of the ice and water shield to facilitate a future tear-off.

·         Upper Window Note – In case anyone is wondering, the upper window is not blocked by the truss as viewed at 6’ + elevation from either end of the room.

11.  Our Questions:

·         I am looking for any feedback on the technical soundness of our design with respect to moisture mitigation, energy efficiency, structural integrity, etc.

·         Soffit Ventilation – we have soffit spaces that will be boxed with LP SmartSide.  My question is do we need to vent the soffit space if all the insulation is terminated at the outside of the wall top plate?  On one hand, I would expect it to be impossible to seal the boxed soffit, thereby seasonal air will make its way into the space . . . on the other hand, venting the space with a pre-vented soffit product such as LP SmartSide would invite dust and insects.

Thank you.

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  1. Expert Member
    BILL WICHERS | | #1

    1- I personally would design for CZ6 just to be extra safe since you’re close to the edge. It may cost a bit more, but you’re protected and you’ll save some energy over the life of the structure.

    2- writing in your drawing got blurred by the image compression so it’s a bit difficult to read.

    5- you may be able to save some weight in your assembly by using I joists instead of 2x12s. Something to think about.

    6- using HFO blown foam will have less environmental impact, and a tiny bit better insulting value too. It does cost more. Consider lighting with other than recessed cans. Recessed cans cause a lot of problems in roof assemblies like this. There are surface mount LED fixtures available now, and I personally like cove lighting (reflective lighting) which has the benefit of very even light distribution and no need to cut holes in the ceiling or run any wiring in the ceiling.

    7- I would consider the use of 5/8” drywall for less chances of sagging over time. I’m not a fan of 1/2” drywall. That’s just my preference though.

    Have you considered putting all of your insulation on the outside using rigid foam? That would eliminate any thermal bridging and may simply some other aspects of your design.


  2. Jon_R | | #2

    If the cost of HFO is excessive, consider a mix of closed cell spray foam and other insulation (eg, open cell spray foam or cellulose). +1 on designing for CZ6. Also considering a vented design using 2x4s crosswise above the purlins.

  3. GBA Editor
    Brian Pontolilo | | #3

    Mr. Dude,

    Below you will find a link to an article on insulated cathedral ceilings that may offer alternatives to your spray-foam insulating plan. There are lots of ways to approach this and in the end the decision comes down to priorities that may include cost, complexity of installation, environmentally-friendly material selection, etc. Sometimes these are competing values.

    I'm not sure why you are planning to use a peel-and-stick roofing membrane over the entire roof, and I'm not sure how a layer of felt over the membrane will help with a future tear off. Seems like the opposite would be the case and is a method some builders have started to use. They want the protection of the peel-and-stick membrane, but also want to be able to easily remove the entire roof down to the sheathing. So, they install the felt first, then the peel-stick.

    Finally, I always have a bit of concern when users ask on this forum if their plan looks good structurally. I hope this has been approved by an engineer.

    Here's the article that I recommend:

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