Few roofing contractors, if any, get excited when customers are experiencing ice dams during the winter. It's often difficult to explain the many factors that contribute to ice dam formations. It's even more difficult to take immediate action to stop a leaking ice dam because steep-slope roof systems covered in snow often are not accessible. Unfortunately, for contractors in northern climates, modern construction practices are creating complex attic assemblies that are more likely to contribute to ice dam formations. Contractors need to carefully analyze all the issues to determine the root causes of ice dam formations, of which there could be many.
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—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 generally will not cool roof sheathing enough to cause melted snow to refreeze, according to Wayne Tobiasson's article, "Ventilating attics to minimize icings at eaves," published in 1994 and based on research conducted at Cold Regions Research and Engineering Laboratory. 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.