Ice and water shield is a self-adhesive, waterproof membrane installed in the most leak-prone areas of a roof, such as valleys, around chimneys and skylights, and along the eaves. Unlike standard underlayment, it sticks directly to the decking and seals tightly around nails, creating a watertight barrier. It is used as added protection in the spots where wind-driven rain and water backup are most likely to force their way under the shingles.
Standard underlayment covers the whole roof and does a good job, but it is not fully sealed and water can still creep under it in the toughest spots. Ice and water shield is a heavier, sticky membrane made for exactly those spots. It peels and adheres directly to the wood deck, bonding into one continuous surface, and it seals around every nail driven through it so the fasteners themselves do not become leak points.
Roofers concentrate this membrane where water gives the most trouble. Valleys, where two slopes funnel a torrent of water, get a strip down the center. Penetrations like chimneys, skylights, and vent pipes get it wrapped around the base. The eaves and the lower edge of the roof get a course of it as well, since that is where water can back up and pool. In each spot it is the last, fully waterproof line before the deck.
For a homeowner, ice and water shield is part of what makes a roof genuinely watertight rather than merely shingled. Even in a warm climate where actual ice dams are rare, the membrane earns its place by sealing the valleys, eaves, and penetrations against the wind-driven rain that drives most leaks. A quality replacement specifies where it goes, and that is a fair detail to confirm in your estimate.
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