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Ice Dams

As the weather starts to warm and things begin to thaw, you may notice the effects of glaciation on your roof eaves, also known as ice dams. This may also occur in other areas as well. Roof mounted bathroom vents, kitchen exhaust vents, and valleys areas are all likely places for ice dams to occur. During spring time thaw, all of the ice that was forced under the shingles will begin to melt and find a pathway into the home or it may also exit out of the laps or junctions of the siding. It is always a good idea to inspect the perimeter of your home under the horizontal eave lines (soffits) for water infiltration. It is also a good idea to inspect the windows located under the soffits in the inside of the home as water may travel down structural members and exit at the top of the windows. Generally it will collect on the window sills and possibly the carpet below. If you experience any of these issues call Avalanche Construction Inc. as we are experts in water infiltration. We can offer long term solutions to ice dam issues by providing a full evaluatioin of your current conditions. This will pinpoint the cause and we can then provide a remedy!

How Do Ice Dams Form?

Utility Exhaust Vents-

Depending on the location of the ice dam, it may have a different effect on your home. If it is around a bathroom and/or kitchen stove exhaust the heated air from inside the home melts the ice around the exhaust vent. It then freezes and drives the ice upwards under the roofing material and exhaust vent. As this freeze/thaw cycle continues, the ice builds up and causes the roofing materials to heave. When it warms in the spring, the ice melts under the roofing material and the water enters your home.

Valley Areas-

When the freeze/thaw cycle occurs in a valley area, the effect of the ice dam is called glaciation. During the freeze thaw cycles it forces the ice upward under the shingles and/or flashings. As it builds up, it begans to cause the roofing material and/or flashing to heave and the ice dam begins to migrate downward causing a glacial effect and begans to create pressure on either side of the valley. This tears shingles, breaks plumbing vent pipes, and can destroy valley flashings. If this occurs on a metal roof that is not properly designed for snow prone areas, it could literally remove the roof from your home!


When the heat from your home escapes into your attic and heats the underside of your roof it begans to melt the snow and/or ice. The water runs down the roof slope and freezes at the roof overhang as there is no insulated space under the eaves. This freeze thaw cycle drives the ice upwards under the roofing material. As this freeze/thaw cycle continues, the ice builds up and causes the roofing materials to heave. When it warms in the spring, the ice melts under the roofing material and the water enters your home.

Will Ice Dams Damage My Home?

Ice dams have the potential to cause a lot of damage. Depending on the location of the ice dam, severity, and the space below will determine what kind and how much potential for damage. Sometimes it may ruin the ceiling inside the home, create mold issues, and/or ruin the insulating value of your attic insulation. It may go unnoticed if it is leaking out of the siding on the exterior wall and over time create structural damages that can be intrusive and costly. There is a chance that it may de-laminate your roof sheathing or damage roof trusses, joists, or fascia. No matter how you look at the situation, it should be remedied to prevent extensive repair costs in the future.

How Do I Prevent Ice Dams?

Building Design

Ice dams are preventable. It starts with building design. The design of the structure should be in such a way to minimize potential problem areas that are known to aid in the development of ice dams. While design is important, this may not help if your home is already built and it would not be practical nor sensible to rebuild your home because of ice dams.


As heat migrates up into your attic by way of convection due to pressure variances caused by the stack effect it warms the underside of the roof sheathing initiating the start of the ice dams. It is important to have a good thermal barrier to stop the migration of heat. The temperature of the attic space ABOVE the insulation should be realatively close to the temperature outside. The attic insulation should be installed correctly and should be a minimum of R-38.


Ventilation is equally as important as insulation and vice versa. Without one the other will not work properly. If adding new insulation it is imperative to properly install the insulation while providing adequate air flow. This is a calculated value determined by attic area. The ventilation is designed to create a natural thermal convection loop that pulls any moist warm air out of the attic area while bringing in cool dry air from the soffit locations.

Self Adhering Underlayment

Because not everything is 100% preventable, there any many buiding practices that work together to achieve a common goal. The design, insulation, and ventilation all work together to help minimize the ice dam effect. But what happens if it still occurs?

During the roofing or re-roofing process it is code to install self adhering underlayment at all eaves, valleys, gables, and all protrusions. Unfortunately, it is almost always done inefficiently. Why? It costs money, it is labor intensive, and you will never know the difference until it is too late. Going back to doing things above the minimum that is required, it is extremely important to apply that school of thought to your roof or reroofing project. I always recommend three courses (9') of self adhering underlayment into the warm wall at all horizontal eaves. In the event water does get under the shingles, the self adhering underlayent will not allow it to infiltrate the home.


A home is the sum of all of it‘s parts. Everything has to work harmoniously to achieve the desired end result. Many things need to be considered and thought through to truly be beneficial. It takes the desire to go above and beyond the minimums required by code. It also takes a clear understanding of the challenges we face due to the climate in Southcentral Alaska as well as first hand experience in successfully resolving issues.

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