Design detail for a heavy-snow load, cold shed roof?

10 years ago

last modified: 10 years ago

Need advice on the design/structure of a cold shed roof that is still flat (not trussed) on the underside -- but okay if it shows purlins and rafters? This is for a modern ski cabin/chalet. I want to use tongue-and-groove (either planks or plywood) on the underside above any purlins and rafters. I love seeing the structure of the purlins and/or rafters. But the roof needs to be highly insulted (~R50+?) and hold a heavy snow load each season (300 psf).

I know there needs to be vapor barriers, a significant cavity for insulation, and a vented cold air cavity between the insulation and upper, weather-proof roof layer. Pitch will be about 3/12. I figured using something like SIP panels for the insulated portion of the roof would be the right approach, but in order to handle the heavy load, it will almost certainly need engineered beams for the rafters and purlins (glulam? i-beam?), and then not sure what the best way would be to create an air cavity above the SIPs before putting on the final roof decking and shingles or metal.

The shed roof will span walls that are 24' apart at a ~3/12 pitch (~18 degrees), and extend past the walls at the top and bottom by about 10' to protect the exterior of the house from weather. No gutters here. So the roof will be a total of about 44' from the top eve to the bottom eve. Not sure how to incorporate the necessary soffits for airflow. I'd like to cantilever the eves as much as reasonably possible, but recognize they may need additional support from rafters and purlins to carry snow load. Since I'll be permitting the build, I already know I'll need an engineering stamp on the plans, but I'm hoping I can get close to what will be needed on the initial designs for budgeting purposes before taking it to an engineer for tweaking.

Can someone point me to the detail of the various layers and structures I need to insulate well, prevent condensation, provide adequate cold air ventilation, and support the structure and weight? I'm not sure what the *details* of the roofing layers should look like.

Featured Answer

j. KnArrowHaus
Original Author

10 years ago

I had hoped in my initial post that this forum was a place for thoughtful discussion and a free exchange of ideas. Thank you lazy_gardens, JDS, and Sophie Wheeler for disabusing me and anyone happening to stumble upon this topic of that notion. The chastisement for "intellectual inflexibility" is particularly ironic coming from a "PRO"...

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Comments (7)

User
10 years ago
You need a structural engineer who is licensed in your area, and an insulation contractor who does work in your area.

This is way out of the scope of Internet research.
j. KnArrowHaus
Original Author
10 years ago
lazy_gardens - thanks for your concern.

I agree the final details need to pass code and county regulations and to be reviewed by a licensed engineer. With that understanding, I'm trying to get my design close and reasonably workable before passing it off for final review by experts.

I'm hoping there are some folks on this site who have school-of-hard-knocks-experience and can provide structural guidance based on designs that have worked / failed in heavy snow environments.

I certainly have no plans to begin building based solely on internet opinions. But I would love to infuse my design with the experience of those who've gone down this path before.
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User
10 years ago
You are concerned with gravity loads, vapor movement and insulation when the wind is likely to blow this structure down like a house of cards. You need to hire a structural engineer now rather than later.
User
10 years ago
Those who have gone down that path before are called architects and structural engineers. And you desparately need one sooner rather than later. The intellectual inflexibility created by getting attached to unworkable ideas because you've spent so much time on them will kill your project entirely. Or put it so far over budget that it's DOA.
User
10 years ago
"With that understanding, I'm trying to get my design close and
reasonably workable before passing it off for final review by experts."

If I were in your position, building in an area where snow load is significant, I would not go any further than a preliminary floor plan and traffic flow design based on the site I owned (layout and orient for best solar gain and snow control, drift control).

I would certainly not be trying to decide insulation depths and rafter detailing and roof loads: that's what engineering software is for.
mermanmike
10 years ago
I don't have any answers or even thoughts for you regarding your question...I just wanted to say nice job on the CAD model. The design is very cool and I hope you will post updates as you progress. Watching my partner build homes and additions over the years has taught me that almost anything is possible given enough skill and dollars. Good luck