Icicles on the edge of the roof may look attractive -- but they are often the sign of an ice dam building up on the roof.

Ice dams are the accumulated and persistent piles of ice and snow that can form along the edge of your roof - just above the gutter - and prevent the water from the snow that's melting further up from reaching the gutter. They form when the temperature of the roof along and above the eaves is significantly lower than the temperature further up. When that happens, the snow along the edge of the roof doesn't melt as soon as the snow above it does.

It's easier to prevent ice dams than to remove them. Once you've got water seeping into your house, you need to deal with the ice dam. Here's the challenge: you have to balance removing the ice dam and preventing further damage with the possibility of causing damage to the roof as you remove the ice. No solution is ideal. Do as little as you can, and no more than you must.

Here are some ways to deal with the ice dam if there is water seepage. Be cautious: working on the roof or below it can be dangerous in the best of weather, and ice, cold, and snow can make it more dangerous. Roofing materials are more brittle in cold weather, and more susceptible to damage.

In the worst case, the snow along the eaves can not only slow down the water coming down the roof but cause it to re-freeze, creating a solid dam of ice. The water backed up behind that dam can then move under the shingles and through the underlayment into your attic, ceiling and walls.

• Best solution: Find a contractor. This is usually the best solution, as the professional will know what substances are safe and effective to use, and will know how to use ladders safely under these conditions.
• Do not use hammers, picks, axes, or ordinary de-icing salt.
• Set and secure any ladders properly.
• Clear dangerous overhangs and icicles -- being cautious that you position yourself so these and other ice that may become loosened don't fall on you.
• Remove snow from the roof behind the ice dam, to prevent further ice dam buildup and seepage. Use a roof rake or push broom. Use caution to avoid slipping and falling, and be careful not to damage roofing materials.
• Make channels in the ice so the melting snow can run off the roof.
  • If there's a dormer window above the roof, try running hot water from there.
  • Hose the roof with tap water from below on a warm day, just in selected spots, to make the needed channels.
  • Put some potassium chloride or other ice-melting chemical into one leg cut from a pair of panty hose, tie it shut on both ends, and lay this on the roof to melt a channel in the ice dam. Avoid salts which contain oxidizing agents, as these may cause rust and corrosion and may damage roofing materials. This includes sodium chloride (NaCl) and calcium chloride (CaCl₂). Fertilizer material including urea, potassium chloride (KCl) and ammonium sulfate ((NH₄)₂S0₄) are safer though they may work less effectively.

None of these solutions is a permanent solution; if you have a problem with ice dams, you'll need to take action to prevent the problem from repeating.

• Preventing Ice Dams