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In my research about ice barriers, I came across the following…Where ice dams might form along the eave because snow continually freezes and thaws or frozen slush back up in the gutters, the underlayment application in the area of the eaves must be modified to prevent ice dams from forcing water under the roofing.
Two layers of underlayment should be cemented together with asphalt cement from the lowest edge of the roof and continue up the roof to a point that is at least 24 inches (610 mm) inside the interior wall line of the building as shown in the illustration.
The environment within the envelope of the building provides adequate warmth to prevent ice dams from forming above the heated space; therefore, the two layers of cemented underlayment are permitted to terminate 24 inches inside the interior wall line of the building.