Ice Dams


What is an ice dam?

  • What causes ice dams?

  • What causes different roof surface temperatures?

  • Preventing and dealing with ice dams

  • Preventing ice dams in new homes

  • Mold, mildew,and air quality

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. Figure 1 shows a cross section of a home with an ice dam.

Figure 1. Cross section of a one-and-a-half story house with an ice dam.

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.

DEALING WITH EXISTING ICE DAMS

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 DAMSYou 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.THE CUREAll 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.The diagram on this page gives a good idea of what needs to be sealed, insulated and ventilated:

Long-term action:
  • First, make the ceiling air tight so no warm, moist air can flow from the house into the attic space.

  • After sealing air leakage paths between the house and attic space, consider increasing the ceiling/roof insulation to cut down on heat loss by conduction.

Both of these actions will increase the snow load that your roof has to carry because it will no longer melt. Can your roof carry the additional load? If it is built to current codes, there should not be a structural problem. Roofs, like the rest of the home, should have been designed to withstand expected snow loads. In Minnesota, plans showing design details to meet expected snow loads are usually required to receive a building permit. The plans for your home may be on file at your local building inspection office. To help you understand the plans, or if you cannot find plans for your home, you may want to contact an architectural engineering firm. A professional engineer should be able to evaluate the structure of your home and answer your questions about the strength of your roof.

  • Natural roof ventilation can help maintain uniform roof temperatures, but if the long-term actions described here are done effectively, then only small amounts of roof ventilation are needed to maintain uniform roof surface temperatures. If heat transfer has been reduced substantially, then snow will build up on the roof and cover natural roof ventilation systems, reducing attic ventilation rates. Natural attic ventilation systems are needed to dry the attic space and remove heat buildup during the summer.

Mechanical attic ventilation IS NOT a recommended solution to ice dams in Minnesota. It can create other attic moisture problems and may cause undesirable negative pressure in the home.

WARNING!

  • Any person on the roof during the winter or performing work on the roof from below is risking injury and risking damage to the roof and house. It is important to contact professionals to carry out this job.

  • Whenever a house is tightened up, ventilation systems, exhausting devices, and combustion devices must have enough air to operate safely and effectively!

Weatherization contractors, who may be listed under Energy Management and Conservation Consultants or Insulation Contractors in the Yellow Pages, are professionals who can deal with the heat transfer problem that creates ice dams. A blower door test should be used by the contractor you hire to evaluate the airtightness of your ceiling. In addition, they may have an infrared camera that can be used to find places in the ceiling where there is excessive heat loss.

Interior damage should not be repaired until ceilings and walls are dry. In addition, interior repair should be done together with correcting the heat loss problem that created the ice dam(s) or the damage will occur again.

Preventing ice dams in new homes

The proper new construction practices to prevent ice dams begin with following or exceeding the state code requirements for ceiling/roof insulation levels.

The second absolutely necessary practice is to construct a continuous, 100% effective air barrier through the ceiling. There should not be any air leakage from the house into the attic space!

Recessed lights, skylights, complicated roof designs, and heating ducts in the attic will all increase the risk of ice dam formation.

Mold, mildew, and air quality

Moisture entering the home from ice dams can lead to the growth of mold and mildew. These biologicals can cause respiratory problems. It is important that the growth of mold and mildew be prevented. This can be done by immediately drying out portions of the house that are wet or damp. See immediate action steps listed earlier to get rid of the water source. Action needs to be taken to clean the home environment and maintain its air quality.


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