Here's my HO. Build an exterior frame to hold a new sturdier existing roof that is more sloped and overhangs the original vents. I had a shed similar to that at my last house and I was outside knocking the snow off the roof every time we got more than 12 inches of snow. It really didn't hold up that well in Massachusetts winters, so I can't imagine it would fare much better further north. If it were me, I would cover the outside frame with wood boards or siding and build your roosts and nest boxes inside the plastic interior. The plastic walls and floors will be easier to clean and the airspace between the outer plastic wall and the exterior wood would act as an insulator. This would be way less expensive than a new build. Good luck with whatever you decide.
The past winter was the worst in decades. Normally snow on a steep steel roof such as the one shown will come off multiple times on it's own. This past winter the snow slid off only once toward the end, we had at least 3 feet of snow accumulated and this shed came through that just fine.
I agree pre-fab sheds are prone to collapse under snow load, this one did. However they can be modified to avoid this issue if an inexpensive re-use is the path chosen. That was the only point I wanted to make.
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