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

Snow loads are a major problem for many homes in the winter. Different homes have various live loads (temporary load that a roof structure is designed to support). Many homeowners find themselves with snow loads their roofs are unable to handle which leads to leaking and other permanent damages. As you already know, snow is essentially frozen water. That being said, snow loads can vary due to several factors such as height and moisture content ranging from dry to wet.

The water content of snow varies; dry snow has approximately 4% water content, while wet snow has approximately 35% and Ice is about 95%. Currently the average roof is able to support about 20 lbs per square foot. It is important to remember that this varies due to poor construction and cheap building materials.

Homes come into real danger when the snow on a roof becomes very wet or turns into ice. Since a cubic foot of ice almost weighs a cubic foot of water you can imagine the strain that is put on a roof when there are a few inches of ice (since a square foot of 3 inches of waters weighs approximately 16 lbs). It is impossible to tell whether your house was built to withstand heavy snow loads unless you were highly educated in engineering and construction. Therefore the safest move is to have the snow removed in order to save your home from extensive damages.

Being up on a roof can be very dangerous never mind when it is covered in snow and ice. This is why it is very important to hire a professional who has experience in snow removal. Remember it will be very cost effective to solve the problem as soon as possible before really serious damage is done to your home.

Drift

Snow drift plays a major role in roof damage. This is because it displaces large amounts of snow on particular areas of a roof. Snowdrift is the process of wind moving snow and placing it in a different area. The drift generally stops at stationary objects such as walls or changes in the slope of a roof.  This process is quite common and can cause a great deal of stress on a roof.