Roof Snow & Ice Dams...

[B-Laps]

Depending on the age and construction of your building, you might not have any problems.

If you have a decently pitched roof, then you probably won't have many issues due to the snow load. I don't know what the code is designed for, but the steeper the roof the more load it can handle. If you have a flat pitched roof on a ranch, then you might run in to some issues.

As for ice damns, you will have these form when there is inadequate insulation or air flow underneath the roof. A modern roof should be designed to be completely insulated underneath the entire roofing surface or there should be no insulation whatsoever and proper soffit and cap vents to keep the entire roof at the same temperature.

An ice damn will form when you have a roof that is receiving heat from the interior of the building, and then the water melts and flows down the roofing material, if it then hits a colder area of the roof (Like the eaves, which aren't above any living space) the water will freeze. Eventually the mass of ice present will cause the water to backup underneath the shingles causing leaks.

My fathers old cape would get minor ice dams on the southern exposure that would melt after a few days of sun. The northern exposure would get massive ice dams that caused a fair amount of interior damage. You only need to clear the snow 3-5 feet above the cold section of the roof to prevent the ice from getting out of control.

Here is a link that explains a little bit more.

http://www.thisoldhouse.com/toh/phot...557572,00.html