Where is an ice barrier (ice and water shield) required on a roof?

In areas with a history of ice damming, an ice barrier must run from the eave edge to at least 24 inches inside the exterior wall line (IRC R905.1.2).

Where local experience shows ice forming along the eaves causes water to back up under the roof covering, the IRC requires an ice barrier at the eaves. It applies to coverings such as asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate, wood shingles, and wood shakes.

The barrier must be either two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet. It extends from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line of the building.

Whether this requirement applies in a given location is determined by the local jurisdiction based on its climate and ice-dam history.