Heat loss through the roof is the biggest variable. Some houses have extremely effective insulation and ventilation. Other houses, due to their design, are more difficult to insulate and ventilate. The complexity of the roof geometry is another variable. Unheated unattached garages typically never have ice backup problems because there is no heat escaping to melt the snow and the roof geometry is usually pretty simple.
Why do some roofs have problems and others don’t?