It's a matter of engineering and economics. The higher pitched roof has longer rafters and much more roofing area to cover. The longer rafter spans lower there load carrying ability which somewhat offsets the lower allowable snow load, and the bigger roof adds to the dead load. As for economics, more roof area means more shingles which means higher cost.
Ray
I don't think it makes much difference, I lived in Alaska and our house had a very shallow roof slope, while a house down the street had steep slopes, but it seemed they had just as much snow on the roof as we had.
Although a couple of years we had so much snow (over 6') on the roof that we had a gentleman come by and shovel the roof off to relieve the stress on the rafters.
By the way the church I belonged too collapsed from the weight of the snow on the roof during services, luckily no one was hurt.
Rick Jesionowski
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A very general question from someone who lives in the UK. My impression of Maine and the NE generally is that it gets pretty cold in the Wintertime. And it can snow a lot. So why do the standard buildings for the Maine Central have such shallow roof slopes? Wouldn't snow put a lot of weight on the roof that a steeper slope would avoid?