The three culprits causing ice damming are; inadequate or improper attic insulation, poor attic ventilation, and living space air warming the attic space. One or more of these villains can cause the snow on the warm area of a roof to melt, flow down to the colder soffit area and re-freeze causing an ice build-up. Water continues to melt and flow and can back up under the shingles and cause attic water damage, exterior damage and even penetrate into the house interior.
How do you prevent this nasty problem from happening you ask?
1) Well, firstly, removing the ice dam by ice melter or heat tape is recommended.
2) Regularly removing snow on the roof 4 feet from the edge is a must.
3) Ensure there are adequate roof vents and soffit vents to create attic cross ventilation. Add insulation to R40 and block any ceiling penetrations to keep warm air out of the space.
4) Ensure that all exhaust ducts vent to the exterior through the roof. (NOT to the soffit area).
In many situations, there are indicators present, even in summer, to identify if ice damming has been a problem.