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.
What causes ice damming?
Heat and warm air leaking from the living space below melts the snow, which trickles down to the colder edge of the roof (above the eaves) and refreezes. Every inch of snow that accumulates on the roof insulates the roof deck a little more. This keeps more heat in the attic, which in turn makes the roof even warmer and melts more snow. Frigid outdoor temperatures ensure a fast and deep freeze at the eaves. The worst ice dams usually occur when a deep snow is followed by very cold weather. There are many ways to treat the symptoms, but proper air sealing, insulation, and attic venting are the best way to eliminate the problem.
Cold attics make a lot of sense. Here you let the cold outdoor air work for you. Keep the entire attic space as cold as the outdoor air and you solve the ice-dam riddle. Look at the roof of an unheated shed or garage. Ice dams don’t form on these structures because they are unheated inside.
Insulation: Houses in cold regions should be equipped with ceiling insulation of at least R-38 (about 12 inches of fiberglass or cellulose). The insulation should be continuous and consistently deep. The most notable problem area is located above the exterior wall. In existing structures, where the space between the wall’s top plate and underside of the roof sheathing is restricted, special care should be taken to maintain the R-38 rating.
Ventilation: Soffit and ridge ventilation system is the most effective methods to cool roof sheathing. Power vents, turbines, roof vents and gable louvers are also good considerations to maintain cold air flow. Soffit and ridge vents should run continuously along the length of the house. A 2-inch space or “air-chute” should be provided between the top of the insulation and the underside of the roof sheathing in all applications. The in-coming “soffit” air washes the underside of the roof sheathing with a continuous flow of cold air. CAUTION: Be sure to install insulation baffles above the exterior wall to protect the insulation from the air that blows in through the soffit vents.
Air Leakage: Insulation retards conductive heat loss, but a special effort must be made to block the flow of warm indoor air (convection) into the attic or roof area. Small holes allow significant volumes of warm indoor air to pass into attic spaces. In new construction avoid making penetrations through the ceiling whenever possible. But when you can’t avoid making penetrations or when you need to air-tighten existing homes use urethane spray-foam (in a can), caulking, packed cellulose, or weather stripping to seal all ceiling leaks like:
- Wire penetrations
- Plumbing penetrations
- Ceiling light fixtures
- Attic hatches
- Bathroom exhaust fans
- Intersection of interior partitions and ceiling
When a cold attic and roof just aren’t enough
A cold roof isn’t always a perfect solution. During winters with heavy snowfall, you may get ice dams anyway. Or ice dams may consistently form at the foot of roof valleys (the junction where two roofs meet at a right angle), because they fill with windblown snow. And some sections of roof may be impossible to keep cold. That is when you have to call on secondary strategies to prevent ice dam damage.
- Run special adhesive ice-and-water barrier from 3 to 6 ft. up the roof from the edge the next time you re-roof. Ice and water barrier is a type of self-sealing underlayment that adheres to the roof decking and waterproofs it. You shingle over the top of it. It’s required by the building code in most regions now. Adding ice-and-water barrier is an expensive proposition if you have to tear up an otherwise sound roof, but its cheap insurance when you have to re-roof anyway. Ask your local building inspector how far you should run it up the roof in your region.
- Rake the snow off your roof after a heavy snowfall. A snow rake, which is an aluminum scraper mounted at a right angle on a telescoping aluminum pole, is the simplest solution for occasional heavy snows. If you pull the snow down, it can’t melt and form an ice dam. It’s an effective, if tedious, solution, but only for single-story homes. You can’t reach the second-floor roof. (Never use a snow rake when standing on a ladder!) And you have to take care not to break the shingles, which are brittle in cold weather.
- Install heat cables when all else fails. Heat cables are high-resistance wires that you mount on the roof edge in a zigzag pattern and plug into an outdoor GFCI receptacle. They’re ideal in spots where ice dams regularly occur and can’t be stopped any other way. One problem: You have to route the meltwater away. Otherwise, it’ll refreeze in the gutters and along the roof edge. You’ll have to run the heat cable inside a downspout so the downspout doesn’t clog with ice.
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.