An ice and water barrier (sometimes referred to as an ice and water shield) is a roofing material designed to protect against water damage from ice dams, wind-driven rain, and other sources of moisture. Ice and water barriers, made of a thick, rubber-like material, are typically applied in areas of the roof that are particularly prone to water infiltration, such as the eaves, valleys, and around roof penetrations like chimneys, skylights, and vents.

Ice dams form when heat from the attic melts snow on the roof, which then runs down and freezes at the edge of the roof. This creates a dam of ice which prevents further melting snow from draining off the roof. The trapped water then seeps under shingles and into the roof deck and attic, causing costly water damage.

Wind-driven rain is another common cause of roof damage. High winds can push rain under shingles and into the roof deck, leading to water infiltration. Ice and water barriers are designed to act as a barrier against wind-driven rain, providing a physical barrier that prevents water from entering the roof deck.

In short, an ice and water barrier is an important part of any roofing system. It provides protection against water infiltration from ice dams, wind-driven rain, and other sources of moisture. When installed correctly, an ice and water barrier can provide years of protection.