An ice dam is a ridge of ice that forms at the edge of a roof and prevents melting snow (water) from draining off the roof. The water that backs up behind the ice dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas. Because of this, preventing ice dams is critical.

One cause of warm roof areas that lead to ice dams Cold wall-ceiling intersections at eaves where the insulation doesn’t extend far enough.For ice dams to form, there must be snow on the roof and higher portions of the roof’s outside surface must be above 32°F while lower surfaces are below 32°F. For a portion of the roof to be below 32°F, outside temperatures must also be below 32°F, and the temperatures must be sustained over a significant period of time. Roofs with large surface areas exposed to the sun and having small run-off areas are prone to ice buildup. A classic example is a roof with several gables or dormers. Very heavy snowfalls also can create problems: a foot or more of snow on a roof combined with warm winter temperatures can warm the roof and cause snow melt and ice on eaves.

The snow on a roof surface that’s above 32°F melts, and as the water flows down the roof, it reaches the portion of the roof that is below 32°F and freezes, making a dam.

In split level buildings, unblocked stud bays to the attic can cause warming problems where the stud bays continue past ceiling joists to the attic. As the dam grows, it is fed by the melting snow above it, but will limit itself to the portions of the roof that are on average below 32°F. The water above backs up behind the ice dam, remains a liquid, and finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it can flow into exterior walls or through the ceiling insulation and stain the ceiling finish.

Warm areas on roofs can have several causes. Cold wall-ceiling intersections at eaves where the insulation doesn’t extend far enough (Figure A). In split level buildings, unblocked stud bays to the attic can cause warming problems where the stud bays continue past ceiling joists to the attic (Figure B). In two-story buildings, a gap below the kneewall in the second story is a common air leak area. Unless the joist space is blocked below the kneewall, cold air enters the joist space (Figure C).

Complex roof construction can increase the risk of ice accumulation, such as a roof with different surface angles that vary in their exposure to sunlight and drain into valleys. Weather also contributes to the problem: a foot or more of snow on a well-insulated roof, combined with warm winter temperatures, can result in snow melt and ice on roof eaves. In the latter case, applying a sidewalk snow melt product or having a roofer steam the snow off may be the safest means of removing ice. In each case, however, the most important step to stopping ice dams is to seal the hidden air leaks into attic spaces. If the principal cause was “the wrong type of weather”, every roof would have an ice dam. Observing unheated garage or picnic shelter roofs confirms that weather is not a basic cause of ice dams.

In two-story buildings, a gap below the kneewall in the second story a common air leak area. While adding insulation helps reduce ice dams, insulation above R-44 adds little value. Roof vents are also, at best, a partial solution. Some people use power ventilators such as attic fans or other motorized devices, but these devices can depressurize the attic, drawing warm moist air out of the house and into the attic.