Ice & Water Shield Calculator
Calculate exactly how many rolls of peel-and-stick ice and water shield you need for eaves, valleys, low-slope sections, chimneys, skylights and vents. Built-in IRC R905.1.2 ice barrier code reference, climate-zone guidance, and support for every major brand from Grace and GAF to Owens Corning, CertainTeed and IKO.
Quick Presets
Installation Coverage
Sum of all eaves — assumes a 36″ (3 ft) wide roll covers IRC R905.1.2's “eave to 24″ inside the wall” requirement.
A 36″ roll laid down the centerline covers both sides (18″ each side).
Penetrations
25 sqft each
15 sqft each
8 sqft each
Roll, Waste & Cost
Most common — Grace, GAF, Owens Corning, CertainTeed
10% typical, 15-20% for complex roofs
Typical $90–$130 per 200 sqft roll
Roof Coverage Diagram
Blue zones show where ice and water shield is required per IRC R905.1.2.
Enter your roof inputs to see roll counts
Ice Dams, Roof Leaks, and Why Ice & Water Shield Matters
An ice dam is what forms when warm air leaking from the attic melts snow on the upper part of a roof, the meltwater runs down the cold slope, and refreezes at the eave where the roof overhangs unheated soffit. The resulting wall of ice can grow several inches thick in a single cold snap, blocking subsequent meltwater from draining off the roof. That trapped water has nowhere to go except up and under the shingles — and asphalt shingles are not designed to keep out standing water. Once it gets behind the shingles, it soaks the deck, runs down the rafter tails, and shows up as a brown stain on the bedroom ceiling downstairs. In a bad year, a single ice dam event can mean $20,000 in interior damage to drywall, flooring and insulation.
Ice and water shield — technically a self-adhered polymer-modified bitumen roofing underlayment that meets ASTM D1970 — is the layer of last defense against this exact failure mode. It is a 36 inch wide (or 4 ft wide for some brands) peel-and-stick membrane that bonds directly to the roof deck under the shingles. The asphalt-rubber compound is sticky enough to seal around the shanks of every roofing nail that punches through it, so even if water finds its way under the shingles, the membrane keeps it out of the house. It is required by IRC R905.1.2 from the eave edge to at least 24 inches inside the exterior wall line in any region with a mean annual temperature below 60°F — which is most of the northern United States, all of Canada, and a surprising amount of the western mountain states.
Beyond the eave-edge requirement, the code calls for ice barrier in valleys (the inside corners where two roof planes meet and water naturally concentrates), at every penetration through the roof deck (chimneys, skylights, plumbing vents, attic vents, electrical masts), and as a continuous double-course layer on any roof section with a pitch under 4:12. Many premium architectural shingle warranties from GAF, Owens Corning, CertainTeed, IKO and Atlas now require additional ice and water shield coverage to qualify for the longest 50-year and lifetime warranty terms. Calculating exactly how many 200 sqft rolls you need without over- or under-ordering is what this tool does.
IRC R905.1.2 Ice Barrier Code Reference
The International Residential Code, Section R905.1.2, governs ice barrier installation across virtually all U.S. jurisdictions. Canadian builders use parallel provisions in the NBC (National Building Code) and provincial codes such as the OBC (Ontario Building Code), BC Building Code, Alberta Building Code and Quebec Construction Code.
Eave Requirement
Ice barrier must extend from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line of the building, measured along the slope. A standard 36 inch wide roll covers this requirement for a typical wall + 12 inch overhang.
Valley Requirement
A minimum of 36 inches of ice barrier centered on the valley line is required. A single 36 inch roll laid centered on the valley provides 18 inches of coverage on each side, meeting code.
Low-Slope Requirement
On roof sections with a slope less than 4:12, two courses of ice barrier are required across the entire low-slope area. The second course is offset and lapped a minimum of 19 inches over the first.
Penetration Requirement
All chimneys, skylights, plumbing vents and other roof penetrations must be flashed with ice barrier extending 6 inches beyond the penetration on all sides and lapped a minimum of 6 inches up the vertical surface.
Where Ice & Water Shield Is Required by Climate Zone
IRC R905.1.2 keys the requirement to mean annual temperature, not a specific zone map. Below is a practical regional breakdown for the U.S. and Canada.
| Region | Mean Annual Temp | Ice Barrier Required? |
|---|---|---|
| All Canadian provinces | Below 60°F (well below) | Required – eaves + valleys + penetrations |
| Alaska, Maine, Vermont, New Hampshire, Minnesota, Wisconsin, North Dakota | ~30–45°F | Required, often with extended coverage |
| Northern Tier (NY, MA, MI, OH, IL, IA, MT, ID, WY, CO, UT) | ~45–55°F | Required |
| Mid-Atlantic (PA, NJ, DE, MD, VA, WV) + KY, MO, KS, NE | ~55°F | Required in most jurisdictions |
| Upper South (TN, NC, AR, OK, northern TX) | ~58–62°F | Check local AHJ — often required |
| Deep South + Coastal CA, AZ, NM, southern TX, FL | Above 60°F | Not code-required (recommended at penetrations) |
How to Install Ice & Water Shield in 5 Steps
- 1
Install over a clean, dry deck before any other underlayment
Ice and water shield is the very first thing that goes on the deck, before the synthetic or felt underlayment that covers the rest of the roof. Sweep the deck, scrape off any nail pops or shingle debris, and confirm the sheathing is dry. The membrane will not bond to a wet or dusty surface.
- 2
Roll out at the eave with full adhesion
Snap a chalk line, peel back the first 12 inches of release film, align with the chalk and press firmly. Continue peeling and rolling out the full length of the eave. Walk every square foot with a J-roller or a stiff foam roller to achieve full adhesion — air pockets are leaks waiting to happen.
- 3
Install valley flashing centered on the valley line
Lay a 36 inch wide course of ice and water shield centered on each valley, running from the eave to the ridge. Lap the eave course over the valley course at the bottom and tuck it under any adjoining hip / ridge course at the top. Use a roller, not your boot, to press it into the valley angle.
- 4
Flash every penetration with a 6″ lap up the vertical
For chimneys, skylights and plumbing stacks, cut a piece of membrane large enough to extend 6 inches past the penetration on the roof surface and lap 6 inches up the vertical face. Diagonal cuts at the corners allow the membrane to wrap up the curb cleanly. The shingle step flashing then laps over this protective base layer.
- 5
Install synthetic underlayment over the remaining deck
Cover the rest of the roof deck with synthetic underlayment (or 30 lb felt), lapping the bottom edge of the synthetic 6 inches over the top edge of the ice and water shield. This protects the membrane from UV exposure during the short window before shingles go down.
Use Cases
Asphalt Shingle Reroof
Standard reroof on a typical northern-climate house: ice and water shield along every eave, in every valley, and around the chimney and bath vent. A 200 lf eave / 50 lf valley house with one chimney takes roughly 6–7 rolls of 200 sqft membrane.
Pair with Roofing Material Calculator →Shingle Calculator + Bundles
Once you know your ice and water shield quantity, total your shingle bundles and underlayment squares. The two orders go on the same delivery and onto the roof on the same day, in the same lift.
Open Roofing Calculator →Standing-Seam Metal Roof
Metal panels reach 240°F+ on hot summer days, so use a high-temperature (HT) rated ice and water shield like Grace Ultra or Carlisle WIP 300HT for full-deck coverage under standing seam, exposed-fastener and stone-coated steel roofs.
Open Metal Roofing Calculator →New Construction Deck + Membrane
On new construction, the ice and water shield order goes on the same truck as the roof sheathing. Calculate both at the same time so the roofers are not waiting for material when they start the dry-in.
Open Plywood Sheathing Calculator →Pro Tips From Working Roofers
- •Warm the decks before install. Adhesive flow stops below 40°F. On cold mornings, let the deck face the sun for an hour before pulling the release film, and unroll a single roll at a time.
- •Use the full-adhesion roll method. Walking the membrane with a J-roller is non-negotiable. Foot traffic alone leaves air pockets at the shingle nail line that become leak paths after two winters.
- •Lap 6 inches up walls and curbs. Step flashing alone is not enough. Wrap the ice and water shield up every wall, chimney and skylight curb a minimum of 6 inches before any metal flashing goes on.
- •Cut diagonally around chimneys. A single rectangular piece with 45-degree diagonal cuts at each chimney corner wraps cleanly up the curb without bunching. Pre-cut on the deck, then peel and stick.
- •Order one extra roll for every ten. Even with a 10% waste factor, a deep valley or a tricky chimney can eat through material faster than expected. A single spare roll back at the shop is cheaper than a half-day trip back to the supply house.
- •Document brand and lot on the project file. If a leak claim comes in five years later, your warranty company will ask which product was used. Photograph the roll labels at delivery and keep the receipt.
Trusted by Roofers Across North America
“This is the first ice and water shield calculator I have used that actually adds the right amount for chimneys and skylights without me having to do it on a napkin. I quoted three reroofs from the truck this week using nothing but this page and the phone camera.”
“Travaille parfaitement pour les exigences du CCQ. The low-slope double-course option is exactly what I needed for a flat porch addition in Sherbrooke. Ordered 7 rolls, used 7 rolls, no return trip to the supplier.”
“I send homeowners and small contractors to this page when they ask about IRC R905.1.2 compliance. The eave + 24 inches inside the wall guidance is shown clearly and the roll math is honest. Saves me explaining the same thing five times a week.”
“Re-roofed my own 1980s ranch and had no idea how much ice and water shield I would need. Punched in my eave length, valley length, and 1 chimney — calculator told me 6 rolls of GAF WeatherWatch. Used 5 and a half, perfect for a first-timer.”
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