Feb 21, 2017
Ice Damming 101
Why do they happen and how to prevent them
With winter weather on the way concerns of ice damming and massive ice build-up on roof eaves have resurfaced. This article is intended to explain the ice damming phenomenon, and steps that can be taken to prevent it.
During prolonged winter weather, homes can experience massive ice formations on the roof eaves, leakage into the eaves, or even into the house. Gutters can be ripped off from the weight of the ice while water drips into bedrooms, the kitchen, or even the basement. In addition to the inconvenience of leakage into the house, this leakage can collapse ceilings, warp and stain wood floors, stain ceilings and walls, and rot roof eaves.
Ice on roof eaves during the winter is normal and is to be expected with most buildings. Icicles hanging from the gutters can be picturesque during wintertime, and this is unavoidable in cold climates. However, problems arise when there is too much ice.
Heat from the living spaces of the building rises through the structure and melts snow from the roofs. The water from the melted snow freezes on the cold eaves and gutters of the roof overhangs. As this process repeats itself, ice continues to build up. After the gutters become clogged with ice, water will continue to run and freeze over the gutters, forming icicles. The lump of ice on the roof eaves is known as an “ice dam”. The “ice dam” restricts additional water from flowing off of the roof.
Most homes in this area are equipped with asphalt shingles. Asphalt shingles are not intended to be impervious to standing water. They rely on gravity to drain water off of the roof so that there is no standing water or “ponds”. The same is true with slate, cement tile, and terra cotta roof surfaces. As the “pond” builds up at the ice dam, water will seep back under the shingles and leak into the roof eaves or into the house.
Many of us have seen water stains and drip marks on the bottom of roof eaves and on the siding of the house that is directly below the roof eaves (where rain water cannot cause staining). This mostly occurs as a result of leakage due to ice dams. During the winter, when this ice damming and leakage is occurring, you can often see icicles dangling from the bottom of the eaves and on the wall beneath the eave.
Prolonged freezing temperatures (especially several weeks of temperatures below 20 degrees) combined with frequent snowfall frequently lead to “ice dams”. This is due to the snow continually melting from the roof and refreezing on the eaves. These temperature and snowfall conditions are unavoidable, since we have no control over the weather. However, there are other conditions associated with the building that contribute to the formation of ice dams that we do have some control over.
During the wintertime, when heating our homes, heat is always escaping to the exterior. This, along with escalating fuel prices, is the reason why we don’t like opening the monthly heating bill when it arrives in the mail. A significant portion of this heat loss typically occurs through the attic.
The first and most important approach to control ice damming is to reduce the amount of heat loss into the attic, since this will help to keep the bottom of the roof colder. This means adding more insulation and sealing gaps or openings between the heated living spaces and the attic. This includes insulating recessed lights, insulating and weather stripping the attic access door, and insulating the attic side of pull-down ladders. Exhaust from bathroom fans should be ducted to the exterior. These measures reduce the roof temperature and reduce your heating bills.
The second critical step in reducing ice damming is to improve ventilation of the attic space and subroofing. Good air circulation along the bottom of the roof will help to keep the roofing surface colder. Whatever heat is lost into the attic space should be vented to the exterior so that it is not allowed to heat the roof and melt the snow. This is referred to as “maintaining a cold roof”. If the roofing surface is kept at, or near, the same temperature as the outside air, leakage from ice damming will not occur. Improving ventilation typically means installing soffit vents in the roof eaves and roof vents or ridge vents near the peaks.
Since we don’t live in a perfect world, and none of us live in a perfect house, it is often difficult to obtain ideal insulation and ventilation in attic spaces. This is especially true in cape-cod style homes and cathedral ceiling construction with limited space between ceilings and roofs. Because of such difficulties, ice damming can continue to be a problem, especially at the bottom of roof valleys and where roof runoff collects. In these cases, other approaches need to be used to control the problems with ice damming.
Some homeowners will choose to remove the gutters from their house in response to ice damming. Although this removes one impediment of water drainage from the roof (and subsequent ice formation), this can increase dampness in the basement and the risk of frost heave of the foundation walls.
Raking and removing snow from the roof, chopping paths for water through the ice on the eaves, or sprinkling rock salt on the eaves are other ways to reduce the chances of leakage into the house. However, some of these methods can be dangerous and might be better left to professionals. These steps are often undertaken when leakage is already entering the house and causing damage, and may have to be repeated until the weather breaks and the snow is all melted from the roof.
If a new roof is being installed, an ice and water shield underlayment should be installed in the area of the roof eaves and valleys (this is now required by modern codes). However, there are many different qualities of ice and water shield underlayment, and some of these are not very effective. This underlayment will not prevent ice formation on the roof eaves, but it helps to resist leakage into the eaves and the house of water that leaks through the roof shingles.
A sheet metal ice slide roofing surface can be installed on the eaves, since this resists ice buildup and provides a more impervious surface to prevent leakage. These ice slides are typically found in areas where snowfall is more abundant, such as in the Adirondacks, but these are not common in our immediate area. Aesthetic appearances should be taken into consideration for a homeowner that is considering this approach.
The installation of electric heating cables on the roof eaves and in the gutters is not uncommon in this area. These heating cables are typically installed in the gutters and downspouts, and they are also installed in a zigzag fashion on the roof eaves. The heating cables provide a path for melted snow to drain from the roof to prevent “ponding” and leakage. Heating cables can be effective, but they use electrical energy (which increases electric bills), there is some concern that they could pose a fire hazard, and the installation can cause premature wear of the roofing shingles that they are installed over. In addition, there is the cost of installing electrical circuiting and the life expectancy of heating cables is limited.
In summary, ice formation occurs on most buildings during prolonged, cold weather with repeated snowfall. The prevention of ice damming and subsequent leakage should first focus on good attic insulation and good ventilation of the subroofing to maintain a “cold roof”. In some cases, providing adequate insulation and ventilation is not practical and other methods must be used. These include raking snow from the roof, installing ice and water shield underlayment, installing metal ice slides, or installing electric heating cables. These strategies will win battles with ice damming, even though the war on ice damming in our area will not end.