What are ice dams?

Ice dams are the large amount of ice that collects on the lower edge of a roof or in the gutters and prevents melting snow (water) from draining off the roof.  As more rain or melting snow runs down the roof, it meets this mass of ice and backs up, sometimes under the shingles and into the attic or the house.  The water that backs up behind the dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas.

Why do ice dams occur?

Ice damming usually occurs with a significant depth of snow on the roof.  There is a complex interaction among the amount of heat loss from a house, snow cover, and outside temperatures that leads to ice dam formation.  In simple terms, if the attic temperature is above freezing, it warms the roof sheathing which melts the snow lying on the shingles.  This water runs down the roof until it meets the roof overhang, which is not warmed by the attic and will be at the temperature of the surrounding air.  If the air and the overhang are below freezing, the water will freeze on the roof surface and start the ice dam.


Recommendations to help prevent ice damming:

– Check all insulation, sealing and ventilation in the attic

– Close up attic bypasses – most of the heat loss comes from air leaks caused by unblocked walls, gaps in drywall, and cracks around light fixtures, plumbing pipes, chimneys, access hatches and other ceiling penetrations.  Air leaks can be difficult to stop.

– If you’re getting a new roof –  ensure your roofer puts ice and water shield 3′ – 6′ up from the wall line (or request to have the whole roof done)

– Add roof and soffit vents

– Some roof types are difficult to ventilate and you may need to manually rake the snow off your roof after a heavy snowfall (however, be careful as this can be dangerous)

Alternative DIY method for curing an ice dam – The Panty Hose Method by Roy Barnhart

Fill the leg of a discarded pair of panty hose with a calcium chloride ice melter.  Lay the hose onto the roof so it crosses the ice dam and overhangs the gutter.  If necessary, use a long-handled garden rake or hoe to push it into position.  The calcium chloride will eventually melt through the snow and ice and create a channel for water to flow down into the gutters or off the roof.

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