Ice dams are large build-ups of ice, the most visible manifestation being found at the bottom edge of the roof and gutters as icicles.
A small amount of ice may not cause an immediate problem, but if the ice continues to build up, it will block the water flow down the roof, causing water to back up under the shingles and leak from areas such as the soffits, roof and chimney flashings and skylights into your house.
Here’s How it Happens
- Ice dams form when there is a blanket of snow on the roof.
- Heat from the attic melts snow above.
- Water then runs down the roof between the shingles and the blanket of snow.
- When the water reaches the roof overhang (bottom edge of the roof), it encounters an area of the roof that is not getting any heat from the attic so the water freezes.
- As this condition progresses, the ice at the bottom edge of the roof builds up until it blocks water flow and the overflow creates icicles.
- Water backs up and starts to leak into the soffit area, under flashings, around skylights, etc. and eventually into the home.
You would think that un-insulated attics would generate the worst ice dams. In fact, un-insulated attics tend not to create an ice damming problem because so much heat is lost through the attic, that snow melts as it lands on the roof. A snow blanket has no opportunity to form. Any insulation upgrades to an un-insulated attic must be accompanied by air sealing and ventilating the attic.
In some cases, the roof configuration may not be conducive to preventing ice dams, leaving only one option: heating cables. If your home has heating cables, you have to turn them on before you have a huge ice dam. Most heating cables do not have enough power to melt an ice dam once it is formed.
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