For much of the U.S., the possibility of ice dams forming along steep-slope roofs' eaves is enough to require an ice-dam protection membrane in addition to any other roof underlayment local building codes may require. In such cases, NRCA recommends using a self-adhering polymer-modified bitumen sheet underlayment.
In areas where ice-dam protection is required (based on temperature, history and/or local building codes), an ice-dam protection membrane should extend from an eave's edge to a point 24 inches inside the interior face of a building's exterior wall. The NRCA Roofing and Waterproofing Manual, Fifth Edition recommends extending an ice-dam protection membrane 24 inches beyond the interior face of a building's exterior wall if a roof's slope is greater than or equal to 4-in-12 (18 degrees). If a roof's slope is less than 4-in-12 (18 degrees), the ice-dam protection membrane should extend 36 inches beyond the exterior wall.