How to Prevent Ice Dams


When a large amount of snow accumulates on a roof, heat loss from insufficient insulation and poor attic ventilation can melt the snow from the underside. As the water reaches the roof edge, where it's colder, it forms a dam. The dammed water backs up under the roofing and leaks into your home. The best way to prevent thawing from the underside is to maintain a cold roof.

Things You'll Need

  • Roofing Membrane And Related Roofing Materials And Tools
  • GFCI Receptacle
  • Utility Knife
  • Attic-ceiling Insulation
  • Attic-stair Insulation Kit And Weather-stripping
  • Caulk Or Expanding Foam
  • Caulking Gun
  • Door Weather-stripping
  • Electric Heat Tapes
  • Flashlight
  • Insulation Baffles
  • Weatherproof Outlet Box With Weather Cover

Maintain a cold roof

  • The most significant area for heat loss in a room below an attic is usually the ceiling, especially in the area adjacent to the outside wall. Inspect the ceiling insulation with a flashlight, then take steps to maximize it.

  • Maintain an open channel through which cold air can pass from soffit vents to the attic. Attach insulation baffles made for this purpose on the underside of the roof sheathing. The channel also allows you to insulate over the top of the wall and compress extra insulation into this restricted area.

  • Use caulk or expanding foam (available in cans) to seal gaps around pipes or electric cables that penetrate the top plates of the walls in rooms below. Weather-strip the attic door, or insulate and weather-strip pull-down attic stairs.

  • Seal and cover recessed light fixtures (Type IC only) with insulation (see Warning).

  • Keep attic vents open so the heat that inevitably escapes from living areas into the attic can go out the vent before it warms the roof. The size of attic-vent openings typically must be at least 1/150 of the attic's floor area.

Create a second line of defense

  • Install thermostatically controlled electric heat tapes (wires) in a zigzag pattern along the lower roof edge and into the gutters. Installing the wires is easy--they clip onto the shingles--but you'll need to install an exterior, weatherproof GFCI (ground fault circuit interrupter) receptacle outlet near one end of the roof edge. Typically an electrician or homeowner with electrical savvy can tap into an attic outlet to feed the new boxes on the low end of a gable wall.

  • Or accept the inevitable and install a nailable waterproof membrane that extends at least 24 inches (60 cm) up the roof from a point directly above the inside of the exterior wall. A roof with a 24-inch (60-cm) overhang would require coverage at least 54 inches (137 cm) wide. Although the best time to do this is when you're reroofing, you can remove lower courses of asphalt roofing for a retrofit.

Tips & Warnings

  • Cold drafts from electrical outlets in rooms below an attic indicate the presence of an attic opening in the top of the wall.
  • Snow guards keep snow and ice from dislodging, sliding off the roof and damaging the gutter.
  • When you're using an outdoor outlet for more than short-term use, such as powering a tool, you must equip it with a cover that protects it from weather.
  • Insulating light fixtures that aren't Type IC without providing the necessary space between the insulation and the housing causes overheating and fires.

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