Embarking on a new build in Northern Utah (Zone 5). Home will have low slope shed style cathedral roof of a single slope (no odd shapes of any kind). I’ve been really wrestling with what the best roof detail would be. 

Initially I was planning on an unvented roof as Martin Holladay detailed in this article (Assembly 4 or 5)
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

However, I then found and read Joseph Lstiburek’s “Dam Ice Dam” article (found here: https://buildingscience.com/documents/insights/bsi-046-dam-ice-dam) which goes into great detail that in areas with high enough snow load, which he defines as ground snow load of 60 lb/ft (which my area just barely surpasses) that you MUST have venting to avoid ice dams. Even with an ultra insulated roof. 

So now my conundrum. From reading this website I understand that low-slope roofs don’t vent well. Though there is perhaps some disagreement about exactly HOW badly they vent. I would be happy to vent my roof assembly, but  would like to avoid the cost of the “over-roof” that Mr. Lstiburek outlines in his article. I also couldn’t find any local roofers that are familiar with or comfortable with Mr. Lstiburek’s design so it represents a challenge on that front as well. 

The roof is going to utilize 24″ trusses for the span. My hope was that the below attached assembly would work; consisting of facia vents (to avoid pulling in the thermal heating from the siding) leading to a 3.5″ ventilation channel uninterrupted from bottom to top with R-60 of either closed cell spray foam or closed cell spray foam and batts underneath. The goal would be to both prevent ice dams while also not suffer from any condensation issues  at my low slope. I realize that there will be some thermal bridging from the truss members, but am in hopes that the R value of the softwood lumber over the 24″ of the truss would be enough that it didn’t cause problems. Also, to the extent it is an important detail, the roof assembly/vents would be oriented in the same direction as the primary winds in my area so I could reliably count on wind blowing up the canyon during the day and down canyon overnight perhaps “mechanically” ventilating that space, as I understand there will be little stack effect with such a low slope roof. 

Any thoughts and opinions from the education folks here would be appreciated. Is my proposed vented assembly better than an unvented assembly?  If not, any alternative assemblies that I should consider? 

Finally, apologies for my roof detail drawing skills.