How Ice Dams Form and How to Prevent Them

Roof ice dam
Sawayasu Tsuji / Getty Images

To homeowners in snowy, cold-winter climates, ice dams are a familiar sight. An ice dam is a mass of ice that gathers along the lower edge of a roofline where it overhangs the edge of the home.

Ice dams are more than just an interesting phenomenon. Severe ice dams can weigh many hundreds of pounds, compromising the structure of the roof eaves. More critically, ice dams can cause meltwater to back up under the shingles, where it can flow down and ruin ceiling and wall surfaces. If ignored, ice dams can cause serious damage to your roof, gutters, paint, insulation and interior drywall, and other surfaces.

To the uninitiated, ice dams can be somewhat mysterious, as one house may be burdened with huge blocks of ice thickly covering the roof eaves, while adjacent homes have only a bright cap of snow over the shingles with no signs of ice at all. Why is one house plagued with ice while others remain completely free? The answer can usually be found in the attic.

Causes of Ice Dams

Diagram of an ice dam
This diagram by Owens Corning shows how an ice dam forms and how leakage may occur in your home. Owens-Corning

Ice dams begin when snow melts on an upper, warmer part of a roof, then flows down to the colder eave overhang, where it refreezes. As the ice accumulates, it forms a blockage that prevents additional snowmelt from flowing off the roof. The ice now begins to back up under the roof shingles, where it melts again, soaking the roof sheathing and leaking into the attic. There, it soaks the insulation (rendering it much less effective). It may now can leak through the ceiling drywall below and into your living space. In addition to the interior water damage caused, large ice dams can be very heavy and can damage gutters and even present a safety hazard to people below. 

Ice dams are formed only when the space inside an attic along the underside of the roof deck is above the freezing point. As the warm air beneath the roof heats the shingles and melts the snow on the roof, the water flows down the roofline until it reaches the overhanging eave structure and refreezes. Ice dams usually start or worsen after a heavy snow because of snow's insulating properties. The snow layer traps warm air beneath the snow, which causes it to melt. 

Preventing ice dams is a matter of simple thermal physics. If the air in the attic or against the bottom of the roof deck remains cold, it can never melt the snow lying on top of the roof, eliminating the water necessary for ice dams. Preventing ice dams can be done through a combination of: 

  • Ventilation under the roof deck, which keeps colder outside air circulating through the attic and prevents it from warming above the freezing point so it can melt snow on the roof.
  • Insulation in the ceiling below the attic, which will prevent warm air from rising up into the attic space to melt snow on the roof.
  • Blocking any heat sources that may be contributing to high temperatures in the attic.

Preventing Ice Dams With Ventilation

DuPont base diagram
Home-Cost.com 2011

Houses with good attic ventilation generally do not experience ice dams, and the physics are quite simple: By circulating cool outside air in the attic space (or in the rafter spaces just below the roof deck), the roof surface remains below freezing and cannot melt the snow on the roof. Although it's contrary to what many people believe, a cold attic actually means no ice dams, while a warm attic means "homeowner beware."

There are several means of improving attic ventilation:

  • Where the spaces between rafters are insulated, you can create a continuous airflow from the soffit (the underside of the roof eaves) to the peak of the roof. A soffit-and-ridge vent system usually requires insulation baffles installed at the lower side of the roof, above the exterior walls. The baffles hold back the insulation by 1 to 2 inches, creating a channel for air to flow freely past the insulation. Without them, thick insulation can block the air coming in through the soffit vents, eliminating airflow. These insulation baffles must be combined with a ridge vent that allows air flowing up through the baffles a continuous path to outdoor air. 
  • If a soffit-and-ridge system is not feasible or desirable, ventilation can be provided with soffit or gable vents for intake air and several conventional roof vents for exhaust air. As a general guideline, ventilation systems should provide 1 square foot of net free ventilation per 150 square feet of attic floor space. Net free ventilation is the total area of openings in a vent, minus all screening or other obstructions. 

Adequate ventilation of the rafter spaces against the roof deck, or of the attic itself, will keep the roof deck cool enough that it won't melt the snow lying on the roof, preventing roof dams before they can get started. 

Preventing Ice Dams by Insulating

Leaves on a roof
R Sherwood Veith/Getty Images

In addition to improving attic ventilation, the next best method to lower temperatures against the roof deck is by insulating the ceiling in living spaces below the attic or against the inside surface of the roof. If you have an open attic, this means insulating the floor of the attic. If you have a finished second-story in which a finished ceiling is directly against the roof, this means insulating the rafter spaces (in combination with ventilation baffles). These methods of insulating will prevent heat from rising up to the roof deck and heating it to a point where it can melt snow lying on the roof. 

Sealing air channels from the living space below the attic is just as important as the insulation itself. Gaps around plumbing pipes and around chimneys can be a significant source of heat flow into the attic from the spaces below. Seal these gaps as part of the overall insulation strategy. 

The benefit of a comprehensive insulation effort is that it not only helps prevent ice dams, but it also reduces energy costs. But be aware that insulation alone rarely is enough to prevent all ice dams. It needs to be done in conjunction with improved ventilation.

Preventing Ice Dams by Eliminating Attic Heat Sources

It's also possible that you have heat sources in your attic that you're not aware of. Most commonly this occurs when older recessed lights (can lights) extend up into floor of the attic. These can radiate a surprising amount of heat into the attic—enough to heat up the air well above the freezing point, especially where ventilation is poor. Replacing these old light fixtures with modern recessed lights that can be fully insulated will help reduce the amount of heat radiated into your attic.

Other possible sources of heat include uninsulated HVAC ductwork, vents from clothes dryers, or improperly vented bathroom or kitchen exhaust fans. All of these heat sources can be wrapped in fiberglass insulation to reduce the thermal transfer of heat into the attic space.

Preventing Ice Dams With Electric Heat Cable

Improving ventilation and reducing heat sources against the inside roof surface are the best ways to prevent ice dams, but if these methods aren't practical, then it is possible to install electric heat cable along the edge of the roofline and gutters. 

When looped in a zig-zag pattern along the edge of the roof, heat cable will prevent the melting water from cooling enough to freeze when it reaches the eaves. Instead of freezing, the meltwater flows harmlessly to the ground.

Heat cable is rather unattractive on a roof, but it does serve to prevent ice dams when installed properly. But be aware that mixing water and electricity always comes with risks. Make sure to follow the manufacturer's directions precisely, and maintain the heat cable properly to ensure that it remains safe. 

Battling Ice Dams

Steam ice dam removal
Absolutely Clean Window Washing

Preventive efforts are best conducted during the spring, summer, or fall, but there are also several options for battling them during the cold months: 

  • Keep gutters clean.  Get rid of all those fall leaves before the snow comes. Also, make sure your downspouts are functioning properly. Melted snow has nowhere to go if the gutters are clogged.
  • Use a roof rake after heavy snowfalls: Ice dams appear quickly after a heavy snow because of the insulating properties of snow. Using a long-handled roof rake to removing the snow from at least at lower 4 feet of roof edge can help prevent ice dams from forming. This is the only safe way to remove snow from a roof; never get onto a roof to remove snow in the winter. When using roof rake, use light pressure to avoid scraping the shingles too hard. 
  • Use calcium chloride or an ice-melt product: If you have an ice dam forming, you can apply calcium chloride or a similar product to the ice. (Forget any advice you may have heard about putting salt in pantyhose; it does not work very well, takes a lot more salt, and results in the pantyhose ripping apart). 
  • Chip off the ice: Although climbing onto the roof is never advised, you may be able to chip away some of the ice by hand if you can reach the eaves with an extension ladder. It is usually not necessary to remove the entire ice dam, since opening up a channel may be enough to allow additional meltwater to flow off the roof. This method will require repeated work, though, as the ice will quickly build up again and block the channel you have cut. A chisel, ice pick, or small hatchet are the best tools for this work but don't attempt it if the ice dam is too high to reach safely. 
  • Hire a professional: When ice dam buildup is too much for you to handle, the solution is to call in a firm specializing in this service. Some roofing contractors offer this service during the winter when there is little roofing work to do. Professionals are insured and will remove your ice dam using special equipment such as a high-temperature/low-pressure steamer to melt the ice and snow from your roof. While this service may cost a few hundred dollars, it can prevent significant damage to your home. Do not hire a contractor that uses a high-pressure power washer with a steam box, which can damage shingles.