Eliminating ice damming requires thoughtful preparation
Describing the ice dam process does not need to be complicated; in fact, the definition is quite simple. Ice dams occur when the roof sheathing may have temperature differences from one roof section to another. Historically, roof sheathing at the eaves typically is colder than sheathing located over heated spaces. Ice dams do not occur every time it snows, but they occur when a temperature differential of about 10 degrees Fahrenheit or more exists down the drainage plane of roof sheathing, enabling a thaw-freeze process to occur. Ice dams are most likely to occur when the outside ambient air temperature is 22 F or slightly colder.
Temperatures warmer than 22 F will generally not cool roof sheathing enough to cause melted snow to refreeze. Also, when outside temperatures are significantly colder than 22 F, ice dams are less likely to occur because the roof sheathing usually is below freezing in all areas. It’s important to note ambient attic air temperatures and roof sheathing temperatures will differ. For the sake of analyzing ice dam formations, focusing on the roof sheathing temperatures is more important. Infrared cameras and thermometers are excellent tools to measure roof sheathing surface temperatures.
In the absence of an infrared camera, analyzing snow-thaw patterns can be a great visual of roof sheathing temperature differentials. Photo 1 depicts an unheated detached garage with snow on the roof and an inefficiently heated house. Only the snow on the heated house has melted because the roof sheathing temperature is obviously above freezing while the garage sheathing temperature is close to the ambient outdoor air temperature. Refreezing will not occur and cause an ice dam on any section of an unheated structure mainly because the roof sheathing temperatures will be consistent.
In the past, industry professionals have identified several causes for ice dam formations, including solar radiation, poor ventilation, inadequate insulation, air leakage and mechanical equipment in attic spaces. In reality, some of these items are contributing causes while others are essentially nonfactors. To address ice dam root causes, instead of merely trying to repair a leak from a roof’s surface, let’s analyze these items individually to determine their effects on ice dams.
Although many people believe sunlight, a form of solar radiation, contributes to ice dam formations, it doesn’t. Proof by contradiction can be used to show why sunlight is not responsible for ice dam formations. Making the assumption solar radiation causes ice dams, you should expect that nearly every roof system exposed to sunlight would form an ice dam, including unheated buildings. You also would expect ice dams to not occur on cloudy days or at nighttime. Experience tells us this assumption is false because all roofs exposed to sunlight do not form ice dams, unheated buildings never form ice dams, and ice dams form and actually are more likely to form on cloudy days and at nighttime.