Ice and water shield is mentioned in almost every roofing contract but understood by almost no homeowners. It's one of the most important components in a roofing system — and one of the most commonly skipped by contractors looking to reduce costs on a bid. Here's what it actually does, where code requires it, and how to know whether your roof has it.
What Ice and Water Shield Is
Ice and water shield (IWS) — also called ice and water barrier, self-adhering underlayment, or by brand names like Grace Ice & Water Shield, Certainteed WinterGuard, or GAF WeatherWatch — is a rubberized asphalt membrane with a self-adhesive backing that bonds directly to the roof deck. Unlike felt or synthetic underlayment that relies on lapping and mechanical fasteners, IWS bonds to the deck surface and seals around fasteners that penetrate through it.
This self-sealing property is what makes it effective: even if a nail or screw goes through the membrane, the rubberized asphalt closes around the fastener and maintains a watertight seal. Standard underlayment does not do this.
What Ice and Water Shield Protects Against
Ice Dams
The "ice" in the name refers to the product's primary original purpose: protecting against water infiltration from ice dams. When an ice dam forms at the eave, meltwater backs up under shingles and would penetrate a standard felt or synthetic underlayment. IWS adhered directly to the deck forms an impenetrable barrier at the eave — water backed up by the ice dam cannot get through the membrane into the structure.
Wind-Driven Rain
High winds can drive rain horizontally, forcing water under shingle laps that would easily shed vertically-falling rain. IWS at valleys and eaves protects against this infiltration mechanism — particularly important in hurricane zones, coastal markets, and tornado alley where wind-driven rain is a regular event rather than an exception.
Valley Water Concentration
Valleys are the highest-flow areas on any roof — both roof planes drain into the valley channel, concentrating water flow significantly above what any field shingle area handles. IWS in all valleys provides a secondary barrier against the higher hydraulic pressure that develops in valley channels during heavy rain events.
Where Code Requires Ice and Water Shield
The International Residential Code (IRC) requires IWS in locations where ice dam formation is possible — specifically where the roof surface temperature can go below freezing while indoor temperatures are maintained above freezing:
- Climate Zones 1–2 (Southeast, Gulf Coast): IWS not required by IRC (though still beneficial for wind-driven rain)
- Climate Zones 3–4 (Mid-Atlantic, parts of Southeast): IWS required at eaves — minimum 24 inches inside the exterior wall line
- Climate Zones 5–7 (Great Lakes, Plains, Mountain West, New England): IWS required at eaves — minimum 24–36 inches inside the exterior wall line depending on jurisdiction
Many local codes exceed the IRC minimum. In Minnesota, Wisconsin, and similar markets, local code may require 48–72 inches of IWS coverage from the eave. Check your local jurisdiction's requirements.
Regardless of code, best practice is IWS at all eaves and all valleys on every re-roofing project. The marginal cost is modest; the protection is real.
Products and Performance
| Product | Manufacturer | Type | Notes |
|---|---|---|---|
| Grace Ice & Water Shield | GCP Applied Technologies | Granular surface | Original product; widely trusted; standard specification |
| GAF WeatherWatch | GAF | Granular surface | Common in GAF system warranty installations |
| CertainTeed WinterGuard | CertainTeed | Granular or smooth | Good performance; CertainTeed system compatibility |
| Owens Corning WeatherLock | Owens Corning | Granular surface | OC system compatibility; solid product |
| Henry Blueskin | Henry | Smooth self-adhering | Also used in wall air barrier applications; good deck adhesion |
The granular-surface products are easier to walk on safely during installation (less slippery) and are the standard for eave and valley applications. Smooth products are occasionally used in specific applications where overlap with other materials is required.
How to Verify Your Roof Has Ice and Water Shield
If you're having a new roof installed, include IWS in the written contract with specific coverage areas specified. "Ice and water shield at eaves and valleys" should appear as a line item with the product name.
If you're buying a home or assessing an existing roof, verification options:
- Ask for permit documentation from the last roof installation — permit inspection records often confirm IWS installation was inspected
- If accessible, check at the eave edge where the first course of shingles lifts — the dark granular surface of IWS is visible beneath the shingles in the first 3–4 feet from the eave
- A professional inspection can often verify IWS presence at accessible eave and valley locations
IWS costs $0.20–$0.35 per sq ft of coverage area. On a full re-roof, proper eave and valley IWS coverage might add $400–$800 to the material cost. It's one of the first things contractors remove when competing on price. If a bid comes in significantly lower than competitors on the same scope, ask specifically whether IWS is included, what product, and what coverage area.
- IWS at all eaves: coverage to 24"+ inside the exterior wall line (36–48" in cold climates)
- IWS in all valleys: full valley coverage from eave to ridge
- IWS at all pipe penetrations and skylight perimeters where accessible
- Named product in the contract (Grace, GAF WeatherWatch, OC WeatherLock, etc.)
We include IWS at all eaves and valleys as a standard specification on every re-roof project. Get a free estimate with full written scope or call (800) 555-0100.