It kind of depends on the age of your roof, the roof materials and the pitch. So i live in an old farmhouse with a 1.5 story setup, back when we had a shingled roof, snow wouldn't shed off of it, then if there was a fair amount of snow 2-3 feet it would pile up on the roof. Then ice starts to form underneath the snow from the heat of the house. Plus then sometimes (like as I'm typing) it rains, all that snow and ice on the roof soaking up water can get heavy enough to make an older roof sag especially on outbuildings. Also with the freeze thaw on a shingle roof it shortens the life of the shingles. Now we have a standing seam metal roof, the snow sheds right off and I never think about shoveling my roof again.