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  • Vittorio Gianni

Ice Dams

Updated: Jul 22, 2020

What is an ice dam?

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 dam can leak into a home and cause damage to walls, ceilings, insulation, and other areas.


What causes ice dams?

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. For ice dams to form there must be snow on the roof, and, at the same time, higher portions of the roof's outside surface must be above 0°C while lower surfaces are below 0°C. For a portion of the roof to be below 0°C, outside temperatures must also be below 0°C. When we say temperatures above or below 0°C, we are talking about average temperature over sustained periods of time.


The snow on a roof surface that is above 0°C will melt. As water flows down the roof it reaches the portion of the roof that is below 0°C and freezes. Voila! - an ice dam.


The dam grows as it is fed by the melting snow above it, but it will limit itself to the portions of the roof that are on the average below 0°C. So the water above backs up behind the ice dam and remains a liquid. This water finds cracks and openings in the exterior roof covering and flows into the attic space. From the attic it could flow into exterior walls or through the ceiling insulation and stain the ceiling finish.

Nonuniform roof surface temperatures lead to ice dams.


How can you deal with an ice dam?

There are two avenues of attack: dealing with an existing ice dam and preventing one in the first place.


1. Remove the ice dam by breaking it free in small chucks. Do NOT use an ax or other sharp tool! You’ll cut through the shingles. Instead, tap lightly with a blunt mallet. This is slow, dangerous work, so hire someone experienced at roofing. Even if you do it safely, the chunks of ice can take pieces of shingle with them.


2. Clear out gutters and downspouts. Again, this is ladder work and an easy way to damage either plastic or metal gutters and spouts.


3. Melt troughs through the ice dam with calcium chloride ice melter. Do NOT use rock salt! It will damage paint, metals, and plants beneath the eave and wherever the salty water drains. A good trough-maker is a tube of cloth (a leg from an old pair of panty hose works well). Fill it with calcium chloride, tie off the top, and lay it vertically across the ice dam. It will slowly melt its way down through the dam, clearing a path for the underlying water to flow free.


Preventing Ice Dams


You can scrape snow from the roof whenever it falls, using a snow rake from below or a broom or plastic shovel from above.


BE CAREFUL: The first method can bury you in snow, while the second can send you slipping off the roof. Hire someone who knows how to use a safety line.You can replace your shingle roof with standing seam or other metal roof. Or you can replace the bottom three feet or so of your shingle roof with a wide metal drip edge. Whatever you do, install a water-repellant membrane under any new roofing.


NOTE: If your roof is not very steep, an ice dam can still form on metal roofing and drip edges.


All of these methods treat the symptoms, not the underlying problem, which is the warm roof, caused by poor insulation and venting of the space under the roof. We have found that the only way to cure an ice dam – and prevent one in the first place – is to:


1. Seal all points where warm air leaks from the living space into the spaces immediately below the roof sheathing.


2. Insulate the living space well enough to prevent conduction and convection of heat through the ceiling.


3. Vent the space between the insulation and the roof sheathing, so any heat that does leak through is carried away.