The problem that causes ice dams is not actually the roof, but the attic. If your attic is too warm it heats up the roof deck and causes the snow to melt. The secret to preventing this is to lower the temperature in your attic to match that of the roof.
There are two ways to do this, and we recommend using both of them. The first is to install a balanced ventilation system that provides intake air and exhaust. This provides a path for cool, dry air to enter the attic (usually near the floor of the attic) and the warm, moist air to exit near the peak.
The second step is to install adequate insulation on the floor of your attic. Standard insulation should not be directly attached to the underside of the roof deck. The proper amount of insulation will keep warm air from your home from rising up into the attic and heating the roof deck, thus causing snow on the roof to melt.
The time to take these steps is before the snow falls. Ice dams can form without you even knowing it and may not become obvious until the damage is done.
That snow on your roof should stay up there a long time and melt very slowly because, with proper ventilation and insulation, your roof deck is now cold.