Ice Damming: How does this happen?

What is an ice dam?
An ice dam is a ridge of ice that builds up along the edge of a roof. The ice creates a dam that backs water up and under the roof shingles. Once the water is deep enough, it penetrates the roofing system and creates water damage inside the home.
To recognize an ice dam, look for a bulge of ice attached to the eaves of overhang of a roof. There may be icicles hanging from the edge, and you may see stains on the siding. The rain gutters may be overflowing with ice.   
Often, the bulge of ice is covered with several inches of snow so you may not see it. Under the buildup of frozen snow is the melting snow and ice – water that is entering your home. If interior damage has already occurred, you will see a wet ceiling and wall or water flowing into windows.
How does it happen?
Ice dams begin when snow accumulates on the roof.  Once it builds up on the roof, it acts as an effective insulator. Heat from the attic warms the underside of the roof and melts the bottom snow into a slush/ice/water mixture. This mixture slides under the snow cover and runs down the roof until it meets a cold surface like the overhang.  The slush then refreezes. As more slush accumulates, the layer becomes thicker and thicker, creating an ice dam.  All of this action is hidden from view under the snow cover.

Once the ice dam is high enough to overcome the pitch of the roof, water seeps under asphalt shingles. Standard roof shingle construction is not designed to resist the attack of water pooling on its surface. The alternate freezing and thawing that occurs under these conditions can increase the magnitude of roof leaks. Once the water has penetrated the shingles, it flows under siding and eaves and leaks through the framing of your home.
How can this be prevented?
To prevent ice dams, your first step should be to eliminate heat buildup in the attic space below the roof deck. You can do this by improving insulation, stopping air “bypass” and increasing ventilation. The temperature in the attic space below the roofing should be nearly the same as the outside temperature.
You may be tempted to use electrical snow-and-ice melt cables, but often these just move the ice dam further up the roof.  The cables are hard to maintain, and often they’re unsightly. They must be fastened by putting new holes in your roof – not a good idea!!
It’s possible to remove snow from the problem areas before an ice dam develops if you can reach the area from the ground or the roof edge.  You can buy a long-handled snow rake and carefully remove snow.  Remember, though, that you could easily damage the frozen roof, and it’s a cold and dangerous job.
Where can I get more information?
If you have questions or concerns, consult a professional. Improper insulation or sealing around light fixtures, metal flue pipes, and chimneys can create a fire hazard. Also, ventilation can be tricky to achieve. In general, attics are not friendly places and one misstep can lead to you falling through the ceiling.
   
* Content derived from Mr. Fix-It Home Tips, The Ice Dam Cometh! What’s a Homeowner To Do?       

 

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