Icicles can be beautiful, but when they're hanging from your gutters, they could be a sign of costly heat loss and damage to your home. Those icicles are created from ice dams, a thick ridge of ice that forms around the edge of your roof, building up along the eaves.

This ice buildup prevents melting snow from draining off the roof. When this happens, water can leak into your home, causing everything from warped ceilings to soggy attic insulation — the perfect environment for mold and mildew.

Running hot and cold

Ice dams form when parts of the roof warm up, but not the edges (or eaves). Snow melts on the warm roof and then freezes when it reaches the cold eaves.

Why does the roof warm up in certain areas and stay cold in others? It's caused by heat leaking from your house into the attic and onto the roof. Heat is transferred through leaks in the attic, exhaust fans in the kitchen or bathroom and fireplace chimneys, to name a few.

When an ice dam appears, water backs up behind it, and then it flows under the shingles and into the house. If you see icicles along the edges of your roof, it's often a sign you have an ice dam problem.

Lose the heat loss

Stopping ice dams begins with eliminating heat loss and keeping your entire roof the same temperature. Here's how:

• Increase ventilation in the space between the insulation and the roof sheathing to carry away any heat that leaks through.
• Add insulation to your attic to prevent the conduction and convection of heat through the ceiling.
• Seal any areas where warm air may leak from your living space into the attic, such as a poorly fitting hatchway or pull-down staircase.

If ice dams form before you've had a chance to take preventive measures, carefully remove the snow from your roof using a rake designed for the purpose. Roof rakes should be available at your local hardware store or DIY retailer. If water is leaking into your house, create channels in the ice dam so water can drain off the roof.