As someone new to a cold climate I am still learning the ways of living and homesteading in the cold and snow. Hopefully this little tidbit will help others.
It turns out that snow collects at the intersection of two faces of a roof. On a metal roof snow slides of fairly easily on warmer days but in the corner intersection the snow doesn't slide down .... at all. I'm pretty sure this is the case for any roof material, but the contrast might not be as obvious in a shingled roof.
This is pretty dangerous because as the snow collects over time it gets heavier and heavier and can eventually cause structural damage.
The solution is using an aluminum roof rake (sorry I didn't take a picture, just google it if you dont know what it is) to pull the snow off the roof.
Raking the roof gets harder and harder every time you have a thaw and then freeze. Or even if the temperature doesn't get over freezing your roof might let off some heat, melt some snow and then refreeze it. The layers of ice that build up get harder, and harder to remove.
As you can see in the photo there is some ice left on the roof that I couldn't rake off. This is because I didn't get to it right away when we had a thaw and then it froze again. However I stayed on top of it after that and was able to get LOTS of snow raked off over several iterations. If I didn't stay on top of it, the ice you see would be a block at least a foot tall. And winter isn't over yet!
So stay on top of your roof raking, or better yet... invest in a home with simple roofs! This is another reason why I am a huge fan of simple roofs without any intersections.
Roof raking the rest of the roof can also be important, but not necessarily as critical, so that snow doesn't build up in general, anywhere on your roof.