Ice & Water Shield Calculator
Calculate exactly how many rolls of ice and water shield you need for eaves, valleys, and rakes — based on your roof dimensions and climate requirements.
Where is ice & water shield required?
In many cold-climate areas, code requires an ice barrier at the eaves. A common requirement is for it to extend from the roof edge to at least 24 inches inside the exterior wall line, but exact requirements can vary by local code. Always verify with your local building department before ordering materials.
How eave coverage is calculated
Coverage depth is measured from the exterior wall plate inward across the roof. A 24-inch requirement means 2 feet past the wall. For a standard 36-inch-wide roll, 2 courses covers most eave requirements. In extreme cold climates, 3–4 courses may be required.
Valley installation tips
Valley ice and water shield should be centered in the valley and extend at least 18 inches on each side — giving 36 inches total coverage width. For steep valleys or complex angles, 24 inches per side (48 inches total) is better practice.
Ice and Water Shield vs Underlayment
Ice and Water Shield is a self-adhered waterproof membrane used in the most vulnerable roof areas, like eaves, valleys, and around penetrations. Standard underlayment covers the larger field of the roof deck, while Ice and Water Shield is used where extra leak protection is needed. Unlike basic underlayment, Ice and Water Shield can self-seal around nails and helps protect against ice dams and wind-driven rain.
How to install ice & water shield — step by step
Ice and water shield is a self-adhering membrane, which means installation technique matters. Rushed or improper application leaves gaps that defeat the entire purpose of the product.
- Install drip edge first at eaves. Drip edge goes on the deck before IWS at the eave — the membrane laps over the top of the drip edge. This directs water away from the fascia. At rakes, drip edge installs on top of the membrane after it's applied.
- Clean and dry the deck. Ice and water shield will not bond to a wet, dirty, or dusty surface. Sweep the deck thoroughly and confirm there is no standing moisture. In cold weather, the deck temperature should be above 40°F for proper adhesion.
- Start at the eave edge, work uphill. Align the bottom edge of the roll with the drip edge and peel back 12–18 inches of release film to start. Press firmly to the deck, then continue peeling the film while unrolling across the eave. Always install from the bottom up so upper courses overlap lower ones by at least 4 inches.
- Roll and press for full adhesion. After the membrane is positioned, use a roller or push broom to press the entire surface firmly to the deck, working air bubbles toward the edges. Unbonded areas can trap moisture and lift in wind.
- Cover valleys before the field of the roof. Valley membrane should be installed before field underlayment so it runs continuously without interruption. Center the roll in the valley and press each side down firmly, working from the bottom of the valley upward.
- Seal all laps and penetrations. End laps (where rolls meet end-to-end) require a minimum 6-inch overlap. At pipes, vents, and chimneys, cut the membrane to fit tightly and seal the edges with compatible flashing tape or roofing caulk.
Required eave depth by climate
The IRC and many local codes tie ice and water shield requirements to climate and how far the membrane extends past the interior wall line. Always verify your local requirements before ordering materials.
| Climate | Typical eave depth | Notes |
|---|---|---|
| Cold | 24 in. past wall line | Often covered by 2 courses of a 36" roll. |
| Very cold | 36 in. past wall line | Common choice where extra ice-dam protection is needed. |
| Extreme cold | 48 in. past wall line | Some areas may require deeper coverage at eaves and special details at rakes. |
Always verify with your local building department — some states and municipalities have adopted stricter minimums than the IRC baseline.
Frequently asked questions
Can I use regular underlayment instead of ice & water shield?
No — not where IWS is required by code, and not in valleys regardless of climate. Standard felt or synthetic underlayment is not self-sealing. If a nail, screw, or shingle fastener penetrates it, water can enter freely.
Does ice & water shield go under or over the drip edge?
At eaves, IWS goes over the drip edge. At rakes, the order usually reverses, with the membrane on the deck first and the rake drip edge on top.
What's the difference between ice & water shield and peel-and-stick underlayment?
They are often used to describe the same product category — a self-adhering membrane that bonds directly to the roof deck and self-seals around fasteners.
How long can ice & water shield be left exposed before shingles are installed?
Many products have a limited UV exposure window before they need to be covered. Check the manufacturer data sheet for the specific product you are using.
How do you calculate how much ice & water shield you need?
Measure the length of each eave and multiply by your required depth (2 ft for warm climates, 3 ft for standard, 6 ft for cold). For valleys, measure the valley length and multiply by 3 ft (18 inches each side). Add 25 sq ft per vent or skylight and 50 sq ft per chimney for flashing zones. Add all sections together and divide by 75 to get the number of rolls — or just use the calculator above.