Both snow and ice can have negative effects on your roof.  Homeowners that have homes located in climates where snow and ice are common, should be aware of the dangers and damage excess snow can cause.

Snow loading is the term used to describe the downward force of snow on a structure.  This is a serious concern for people who live in regions that experience a lot of snowfall.  As snow is full of water and very heavy, the roof essentially takes the brunt by taking the weight.   There are certain elements that are more likely to cause damage: improper re-roofing, faulty structure and lack of air ventilation.  Older roofs with weaker structure are at an even greater risk.  The pitch of your roof is also imperative to preventing snow loading.  Flat roofs unfortunately do not have the ability to displace the snow.  The danger is that the more snow is on your roof, the heavier it is and the more risk you have of structural damage.

In addition, snow loads have a tendency to create ice dams, which can cause even more damage.  The best solution in this case is prevention.  Take measures to ensure that snow and ice do not have a chance to accumulate.  Speak to professionals about the best ways to prepare your home for the coming snowfall.

Source:  http://www.doityourself.com/stry/snow-on-the-roof–vs-ice-on-the-roof