What is an Ice Dam?
An ice dam is a wall of ice that forms at the eaves of a building. As this formation of ice grows the ice works its way under the roofing surface. This can cause moisture damage to the sheathing, structural members and ceiling or wall coverings below.
How does an Ice Dam Form?
Generally speaking the main reason ice dams form is due to inadequate insulation and 
air sealing in the attic space. When warm indoor air escapes into the attic space this raises the temperature of the roofing surface causing the snow to melt and flow to the eave. When this moisture reaches the exterior wall the roofing surface is frozen and this water freezes causing the dam. As this ice grows it lifts the shingles creating a condition conducive to moisture intrusion. Air sealing around ceiling penetrations such as plumbing vents and exhaust fans is also necessary to prevent warm air from escaping into the attic.
What are the signs I may have an ice dam?
Some signs of Ice Damming are as follows:
- Large Icicles form at the eave and/or ice forms behind the gutter.
- A thick layer of ice is present on the eave shingles.
- Water or ice appears on the exterior wall.
- Moisture staining is present on the ceiling at the exterior wall.
How to prevent Ice Dams from Forming
The most effective way to prevent ice damming from forming is to winterize your attic. The proper amount of insulation is necessary. A uniform layer of insulation is necessary to prevent any heat loss in the attic space. Sealing any attic floor penetrations, like any plumbing vents or bathroom exhaust fans is also necessary to prevent heat loss. Insulating these pipes and vents may also be necessary to prevent condensation from forming.
Addressing any attic ventilation issues is also needed. Adequate ventilation will help keep the attic cool and prevent snow from melting due to any elevated attic temperatures. Ideally the attic ventilation area should be at least equal 1/300th of the attic floor area. Low-high ventilation is the most effective.
In Summary
Ice damming can cause extensive damage to your home. Annual inspection of your roofing surface and attic area is recommended to identify possible causes of ice damming, moisture intrusion, and areas of inadequate weatherization. Upgrading to current recommended insulation levels generally pays off in the long run due to increased energy efficiency and utility savings.
If you have a home inspection or home maintenance related topic you’d like to read about please send your suggestion to me via email at Aaron@ZuehlkeInspections.com
Aaron M. Zuehlke is the owner and lead inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing and Residential Thermal Imaging.