Ice dams and ice backup

Ice dams form on a roof when snow melts and then refreezes at the roof edges. The cause is an attic that is warmer than the outside air.

Why a warm attic causes a leak

The attic should be as cold as it is outside. Proper insulation is vital, as is proper ventilation — partly from the soffit and partly from the roof vents. It is extremely important that the soffit areas inside the attic are not plugged with insulation, so cold air can enter. It is equally crucial that holes for fans, light fixtures and similar penetrations are properly sealed to stop heat from flowing up into the attic.

Eventually, when there is enough ice on the roof, the tar lines on the shingles let go. When additional melting occurs, water comes into the house. In warm weather those tar lines re-seal, so water does not come in when it rains — which is why the attic issues must be fixed, or the problem returns the next winter.

More vents are not the answer

A good roofing contractor will know how to calculate the correct amount of ventilation for your roof. A common mistake is assuming the attic will be cooler if just one more vent is installed. An additional vent may help, but if there is insufficient insulation and many unsealed holes, it will not make much difference how many vents are on the roof — warm air will keep escaping into the attic, melting the snow so it refreezes.

What we look for

• Heat loss through an under-insulated or poorly sealed attic floor
• Soffit vents plugged with insulation, choking off incoming cold air
• Cathedral ceilings and skylights that block airflow between framing members
• Water backing up under the shingles once the tar lines let go
• Long-term damage to roof sheathing, and rotten eavestroughs