Zone 5 Indiana
Researching new build [New construction] if practical.

Modern build. Roof the [middle/central] section has 1 1/4" slope shed roof with clerestory windows under on both sides. Would like to do unvented roof assembly.
Left & right side wings are gables (1 3/4" in 12)... again unvented assembly. I doubt (low pitch roof can vent).

Want to avoid wet roof deck. I have seen in the past another modern house built in Oregon with R30 bats and the roof decking looked like a disaster.
1) Can this structure be built in northern climate.
2) Roof assembly details: OSB, ZIP roof, underlayment, rigid over decking?
3) Would like to use either 2x12s or I joists.....

Any opinions?

You want to talk to an engineer to make sure such a roof can maintain a snow load properly.
DOUBLY important if you're going to use exterior insulation on the roof. As you will massively decrease snowmelt from interior thermal leak.

Honestly, I'd go with ZIP sheating (well taped and liquid-flashed over ALL nail penetrations).
I'd go with a peel & stick underlayment atop that, rather than roofing felt.
And, because of the low pitch, I'd probably make the first two courses water & ice dam membrane.

Also, recommend making your building a simple box. This makes it easy to tie the wall and roofing weatherproofing systems into one another, sealing the building efficiently.
Then build any overhangs as architectural detail.

If you want to see a high-end version (see "MO MONEY MO MONEY MO MONEY"), watch these videos.


https://youtu.be/yTbMVdmnNlw

https://youtu.be/IDFSnjuNl5U


Keep in mind, this guy builds multi-million dollar places in the Houston area.
Also keep in mind this guy is getting materials either outright DONATED or at a significant discount from the vendor. So there's exactly ZERO chance you can do it as inexpensively as he can.

But the building science he's imparting is good.