Preventing Ice Dams

A Few Preventative Maintenance Tips for Preventing Ice Dams

By Mark J. Donovan




Ice dams can cause a great deal of damage to your home. They can ruin shingles, damage wood roof sheathing, and cause water damage inside the home. Fortunately ice dams do not need to be an annual right of winter passage. By taking a few basic home maintenance steps prior to the onset of winter you can protect your home from them and their damaging effects.

How Ice Dams Form

Ice dams form due to a combination of accumulating snow on the roof, and warm roof surface temperatures caused by insufficient attic insulation and ventilation.

When the snow that is in contact with the warm roof surface begins to melt, due to the poorly insulated and ventilated attic, the resulting water travels down towards the roof edge. As it nears the roof edge, where the roof surface temperature is colder, the water freezes and begins to dam. The situation is exacerbated with the approach of night, and the associated colder temperatures. As the melting and re-freezing process continues the dam becomes thicker and causes water to back up under the shingles. Eventually the water backs up far enough that it literally gets behind the backs of the shingles. When this occurs the water works its way onto the wood roof sheathing, where from there it proceeds to find any hole or crack it can find. Eventually it ends up as water stains on your inside ceilings or even worse.

Preventing Ice Dams

The fundamental solution for preventing ice dams is to maintain a cold attic space and roof surface temperature. Consequently, it is extremely important to properly and sufficiently insulate and ventilate the attic. The end goal should be to achieve an attic air temperature that is nearly equivalent to the outside ambient air temperature. If the outside ambient air temperature is at or below freezing, and the attic temperature is above freezing, snow melt will occur on the roof surface and ice dams will form.

Ice melt socks can be seen at the edge of the upper roof in this picture.

Properly Insulate the Attic

Use at a minimum R-30 fiberglass batt insulation in the attic between the ceiling joists. R-30 batt insulation is 9.5” in thickness. In the process of installing the insulation make sure there are no gaps around ceiling lights, bathroom fans or other mechanical equipment in the attic.

Also make sure that the bathroom fans are properly vented to the outside of the home. If they are not properly ventilated they will send warm moist air into the attic which will lead to the formation of ice dams. Improperly ventilated bathroom fans can also lead to the formation of attic mold and mildew.

Properly Ventilate the Attic

It is important to make sure that any heat that does work its way from the home’s main living space into the attic be ventilated out of the attic as quickly as possible. To achieve this make sure the gable vents, ridge vents and soffit vents are not blocked via insulation or other obstructions. The use of attic rafter vents, along the edge of the roof line can help to ensure proper airflow from the soffit vents into the attic area.

New Construction – Best Time for Preventing Ice Dams

During the construction of a new home, or re-shingling a roof, there are a number of preventive steps that can be taken to prevent Ice dams.

Asphalt Shingle Roofing Bid Sheet

First, the longer the width of the roof overhang over the exterior walls the lower the potential for water damage to the home. Water damage is limited to only the exterior soffit area.

Second, when shingling a roof an ice and water shield underlayment membrane should be used along the edge of the roof, in roof valleys and around plumbing stacks, chimneys, and skylights. It is applied directly to the wood sheathing prior to the installation of the shingles. The membrane is designed to seal around the roofing nails as they penetrate the membrane.

With these few basic preventive home maintenance and construction tips you should be able to prevent ice dams and their damaging effects this winter.


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