Design detail for highly-insulated cold shed roof to carry heavy snow load?

[knopfarrow]

Does anyone have detail or advice on how to design/structure a cold shed roof that is still flat (not trussed) on the underside -- but okay if it shows purlins and rafters?  I want to use tongue-and-groove wood (either knotty planks or plywood) on the underside above any purlins and rafters -- infact, I love exposing and seeing the structure of the purlins and rafters.  But the roof needs to be highly insulted (~R50+?) and hold a heavy snow load each season. 

I know there needs to be vapor barriers, a significant space for insulation, and a vented cold air cavity between the insulation and upper, weather-proof roof layer.  Pitch will be about 3/12. The roof needs to handle 300 psf snow load.  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?)

Needs to span walls that are 24' apart at a ~3/12 pitch (~18 degrees). Also planning to extend the top and bottom eves by about 10' beyond the walls to protect the exterior of the house from weather. So the roof will be a total of about 44' from the top eve to the bottom eve. 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 some of the load.  Since I'll be permitting the build, I'm already resigned to the fact that 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.

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 should look like.

[NathanS]

With that much snow load you are going to have to be very careful with ice damming. Snow has an R-value somewhere between 1 and 2. If you don't have adequate air flow between the insulation and the surface of the roof, temperatures at the bottom layer of snow could quickly get above freezing and start melting, then refreezing at the edge of the roof as it drips down...

Since you want exposed rafters, maybe you could build it like a 'hot roof' with 10-12 inches of layered XPS or EPS (not sure EPS has enough compressive strength), then fur the top layer out 2 inches for your ventilation.

Here are a few articles on roof design -
http://buildingscience.com/sites/default/files/migrate/pdf/PA_Crash_Course_Roof_Venting_FHB.pdf
http://buildingscience.com/documents/insights/bsi-046-dam-ice-dam
http://buildingscience.com/documents/insights/bsi083-mea-culpa-roofs
http://buildingscience.com/documents/insights/bsi-063-over-roofing
http://buildingscience.com/documents/insights/bsi-031-building-in-extreme-cold

Good luck on the structural part, I don't know too much about how you'd design all that.. but, with 10 feet of overhang you may also need to consider uplift from wind. It sounds like there are no interior load bearing walls to help with that 24 foot span?

I liked this resource on how engineers calculate loads and all that -

http://www.huduser.org/portal/publications/residential.pdf


Good luck

[hpinson]

Just curious, why flat?   

[knopfarrow]

Just curious, why flat?

A couple reasons for a shed roof:

• Simplest structure to prevent snow and ice inflitration -- no valleys. All the reading I've done indicates this will be the lowest maintenance and fewest problems over time. (See example pic below!)
• With over 100" average snowpack, doing a shed roof "sheds" the snow to one side allowing for easier access to the cabin on the high roof side.
• I like the modern look and simplicity.