What is an Ice Dam?
An ice dam is a ridge of ice that typically forms along the edge of a roof and prevents melting snow from draining off of the roof.
Ice dams can also form around skylights, vents, and anywhere two inclined sides of the roof meet (i.e., valleys).
Poor insulation and lack of adequate ventilation in the attic space causes the roof deck to heat up and melt the snow above. When it does, the water (AKA melted snow) flows down the roof and gets backed-up behind the dam with nowhere to drain off. Not only does this pool of water eventually refreeze into more ice (and continue to grow in size), but in the meantime the water can work its way under the shingles (or other roofing material). At this point, the water can leak into the home or building, and quickly cause damage to walls, ceilings, insulation and other internal parts of the structure.
Here’s a very basic diagram showing how an ice dam forms on your roof:
Here’s how ice dams form on your roof:
1. Indoor heat rises through the ceiling, into the attic, and warms the roof (from the underside). The shingles get warm.
2. Snow that has fallen on the now-heated part(s) of the roof melts. The melted snow then flows down the roof until it reaches the part of the roof that is lower than 32F (most commonly the overhangs). This water refreezes and begins to form an ice dam.
3. The dam gets bigger and bigger as more snow melts and refreezes. Water now begins to pool behind the increasingly large wall of ice.
4. Eventually, the water works its way through the shingles (or other roofing material) and flows through the roof decking and into the attic. From here it can seep through the ceiling, the interior walls, and into the rest of the building.
(For more on the problems ice dams cause, see our “Why Are Ice Dams Bad?” page.)
This diagram shows in more detail what we’re describing: (click here to open the full-sized image in a new tab):
What now? Assuming you don’t need immediate help to get your ice dam removed, you’ll want to prevent ice dams from forming in the first place. You can browse the other resources in our Learning Center, or read some of our prevention tips.