Ice damming takes place when a warm roof melts the snow and the water rolls down to the unheated portion of the roof such as the edge or gutter, and accumulates there. The roof overhang is especially prone to this due to it being uninsulated underneath. The more intricate roof designs with dips and domes and plenty of overhangs, are more likely to experience incidents of ice damming.
If you have an attic, you can either leave it with no insulation or insulate it well to prevent any parts of it left warmer than the rest. An uninsulated attic will be warmer all over and any snow that touches the roof will melt away as it comes down, minimizing any accumulation.
On the other hand, a properly insulated attic will keep the heat within the house and leave the roof cool so ice would not melt and form an ice dam somewhere.
To spot for ice dams, inspect the roof after the first heavy snow. If you find a section of the roof where snow has considerably melted, check the insulation under that part. You may find that its insulation is inadequate or it has not been insulated at all, causing air to leak.
Source: Cmhc-schl.gc.ca
