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What is ice damming?

What is ice damming, how do I detect it, and what can I do about it?

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Ice dams only occur in climates where snow accumulates on roofs. They are caused by heat escaping from the house and melting the snow on the upper parts of the roof. This melted snow re-freezes when it gets down to the colder eave area where there is no heat loss from the house.

Ice dams allow leakage through the roof into the soffit, fascia and wall systems. Leakage may also develop at the interior of the building, although if it does not, the damage can be concealed and go unnoticed for a considerable period.

Ice dams don't occur every winter. It takes the appropriate snow accumulations and weather conditions to cause ice dams. Snow that accumulates on a roof to considerable depth, followed by temperatures just below freezing for several days, creates an environment where ice damming is likely.

Where Ice Dams Form

Dams are common on low-sloped roofs or roofs which change slope near the eaves. Larger eave overhangs (wide soffits) are more prone to ice dams. Ice dams may also occur where a roof above a heated area extends out over an unheated area such as a porch or balcony.

Ice dams are most common at the lower edge of roofs, but may also occur near party walls (through which there may be considerable heat loss to the roof area) and around chimneys (again, where heat loss may create localized melting of snow and subsequent re-freezing below).

Another spot where ice damming frequently occurs is at the bottom of a valley. Melted snow finds its way to the valley and runs down to the bottom. The concentrated water re-freezing in this area can create an ice dam.

Very often, the northern slope of the roof is more vulnerable to ice damming than the southern section. The heat of the sun helps to melt the snow uniformly across the south roof surface and allow water to run off the edge.

Identifying Ice Damming Problems

It is relatively easy to see ice dams in the winter when there is snow on the roof. Icicles hanging off gutters and protruding from soffits or between siding boards, for example, are a dead giveaway that ice damming is taking place. Similarly, water coming through the wall/ceiling intersection inside along the length of the wall, or near a valley, very often indicates ice damming.

It's much more difficult to identify an ice-damming problem during the summer months. You're less likely to think about ice damming in warm weather. Although some of the physical conditions that allow ice dams to form may exist, it is hard to know whether the problem manifests itself or not. Some of the clues that you can look for include

1. Evidence of damage along the wall/ceiling intersection on the interior. This looks different from a typical roof leak because it extends along the length of the wall for several feet, typically. A roof leak caused by a puncture or missing shingle will rarely lead to leakage that runs horizontally along the wall/ceiling intersection.

2. Discoloration of siding starting at the soffit and running down the wall. It's unusual for the walls to get wet immediately under the eaves. Where streaking is noticed running down from the wall/soffit intersection, ice damming should be suspected.

3. Damage to shingles from axes, hatchets, shovels, etc., along the lower edge of the roof. This indicates that attempts have been made by previous owners to break up ice dams.

4. Electric cables running along the lower part of the roof in a zigzag pattern. These heating cables are often used in an attempt to protect against ice damming. These cables are only effective if they are turned on before the snow accumulates. They won't work if there's a foot of snow on the roof before the cables are turned on. These cables, by the way, provide more opportunity for leakage because they have to be secured through the roof coverings.

5. Roofs with poor insulation and ventilation are more likely to suffer from ice damming problems because their attics will be hotter. A lack of insulation will allow a lot of heat loss from the house into the attic. Poor ventilation will prevent this heat from escaping from the attic. The heat in the attic will melt the snow on the roof above the attic. This melted snow will run down and re-freeze at the edge, creating the dam.

6. The tops of windows may indicate ice damming if evidence can be found of water leakage through the outside or inside of the window top. While it may be leakage, the higher the window and better protected it is by the roof overhang, the more likely it is to be ice damming.

7. You might be able to identify an ice-damming problem from the attic space, although this is rare. The water penetration is likely to be near the roof edges and at the bottom of valleys. This area is often obstructed by insulation. Staining, wet areas or rot on the underside of the roof sheathing along the perimeter for any distance may suggest ice damming.

8. In some situations, the insulation may be compressed from the dampness, or there may be stains visible in the insulation, indicating water has been dropping onto it. Some old insulation has a kraft paper barrier on the top. Stains on the paper may be visible.


One way to determine whether the problem is ice damming or leakage is to monitor the situation seasonally. If the area is wet after a rain, or when there has been no snow on the roof for some time, it is a leak. If the problem only occurs when there is snow on the roof and icicles hanging from the gutters, it is ice damming.

Possible Solutions To Ice Damming

There are several things done to minimize ice damming, including 1. upgrading insulation and ventilation 2. adding eave protection 3. adding electric heating cables One of the best is upgrading the attic insulation and ventilation to keep the attic as cold as possible. This will minimize the snow melting on the roof surface. Good soffit venting is essential here.

Eave Protection

Eave protection is recommended in northern climates where ice damming may be an issue. Eave protection is a waterproof membrane laid along the lower edge of the roof or anywhere ice dams may cause water to back up under the shingles. This includes chimneys, skylights, valleys and party walls.

There are several eave protection materials that are used. These include 1. modified bitumen 2. polyethylene 3. roofing felts or roll roofing 4. metal sheets

1. The self-healing, peel-and-stick modified bitumen products are among the best. These typically come in 3-foot-wide rolls and provide good protection. This material is sometimes generically called Ice and Water Shield, although this is a brand name of W. R. Grace & Co.

2. Polyethylene sheets (6 mil) were used as eave material in several areas, but this has been disallowed in some, since it is not terribly effective. Polyethylene, with nails driven through it to secure the overlying shingles, will not be watertight. Polyethylene also degrades over time. Joints are often not well sealed.

3. Roll roofing or two layers of #15 felt paper, cemented together are sometimes used as eave protection material. These are better than polyethylene, but not as good as modified bitumen.

4. In some areas, the bottom several rows of shingles are replaced with metal. The metal allows snow and ice to slide off the roof, discouraging the formation of dams. The metal is also watertight. Metal is occasionally used as eave protection under the shingles. The drawback to this is that if the shingles are nailed on over the metal, the nail holes allow leaks in the metal sheet.

Building codes have changed over the years as to how far up the roof the eave protection should extend. It is common to find recommendations to carry the eave protection up at least 12 inches past the interior face of the exterior wall. Eave protection should extend at least 3 feet up from the roof edge. A common problem with eave protection is failure to extend the protection down to the roof edge. If the material stops just short, water may be deposited on the roof sheathing just above the bottom edge.

Eave protection is not usually provided if the roof is over an unheated area, such as a garage or porch with a roof extending out more than 3 feet from the house wall. On low-slope roofs (less than 4 in 12) a watertight membrane is usually laid under the shingles in any case, and eave protection is not necessary. If the roof slope is greater than 8 in 12, eave protection isn't usually provided. The thinking here is that the steepness of the roof will prevent ice dams from allowing water to back up under the shingles.

You can identify eave protection from the lower edge of the roof or the bottom part of the rake. If you are in a climate subject to ice damming and see no eave protection on a roof where you'd expect to find it, you may want to note this as a condition that should be monitored, rather than something that must be added.If the house is 25 years old and has an original roofing material with no eave protection, yet you can find no evidence of damage from ice damming, it does not make sense to tear up an existing roof to add eave protection when there is no evidence of damage.

Drip Edge Flashing

Drip edge flashing is used more in some areas than others. This flashing directs water coming off the roof into a gutter or onto the ground. It prevents water from being wicked up into the sheathing or fascia, and is useful in areas susceptible to ice damming. It typically is a roughly four- or five-inch-wide piece of bent metal that extends about three inches up the surface of the sheathing from the edge. This metal surface is covered by the roofing materials. The rest of the flashing extends down vertically, covering the edge of the sheathing and/or fascia. The bottom half inch or so is usually bent out at a 45 angle so that water will drip off the flashing out away from the fascia. In some areas, drip edge flashing is used along the rake as well as the lower edge.

Where eave protection is used, there are two schools of thought as to whether the eave protection should overlap the top edge of the drip edge flashing, or be tucked under it. If you can envision water getting through the shingles and running down across the eave protection, it's better to have the protection extend over the top of the drip edge so water won't be driven in underneath the drip edge. If, however, you envision water backing up from the gutter as a result of ice dams, for example, or water being blown up under the bottom shingle by wind-driven rains, you might want the eave protection to be tucked under the flashing. We prefer to have the eave protection extend over the drip edge flashing, and since it is also our preference to use self-sealing modified bitumen as eave protection, the argument becomes moot since the eave protection seals itself tightly to the drip edge flashing. We should emphasize that the absence of a drip edge flashing is not, in itself, a roofing defect.

Avalanche Guards

Avalanche guards are tabs, spikes or rails that project above the roof surface. They are usually found along the lower parts of the roof, often over entrances. Some people think that avalanche guards help to prevent ice dams. Their real function is to prevent large quantities of snow from falling off the roof at one time. They are common on large roof areas such as churches, municipal buildings or large homes. They are also more common on slate, clay and concrete roofing surfaces.

The guards hold the snow onto the roof, so heavy single blocks of snow and ice do not fall off the roof, causing damage or injury to persons below. Avalanche guards are more important on tall buildings where the snow and ice develop considerable momentum as they fall. Avalanche guards do not eliminate ice dams.
by Manage My Life
April 26th, 2007
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