Ice and Water Shield Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Ice and water shield must extend at least 24 inches inside the exterior wall line, per IRC R905.1.2. For a typical 1 ft overhang and a 6:12 roof, that means roughly two courses of a 36 inch wide roll along the eaves, plus full coverage up each valley, sized from standard 200 sq ft rolls.
Your measurements
Enter the values you know — results update live.
Combined length of all eaves needing ice and water shield.
Overhang plus wall thickness — the horizontal distance from the outside wall face to the eave edge.
Steeper roofs need a longer sloped distance to achieve the same 24 in horizontal inside-wall coverage.
Standard rolls are 36 in wide.
Analysis
Live results from your measurements.
Coverage meets the IRC R905.1.2 minimum of 24 in inside the exterior wall line.
Show the working
- Horizontal coverage needed:
18 + 24= 42 in - Slope factor:
√(6² + 12²) ÷ 12= 1.118 - Slope coverage needed:
42 × 1.118= 47 in - Courses needed:
ceil(47 ÷ 36)= 2 - Eave area:
100 × 6= 600 sq ft - Valley area:
20 × 3= 60 sq ft - Rolls needed:
ceil((600 + 60) × 1.1 ÷ 200)= 4 rolls
What this calculator does
Ice and water shield is a self-adhering waterproof underlayment installed at roof eaves, valleys and other leak-prone areas to stop ice dams and wind-driven rain from backing up under shingles. IRC R905.1.2 requires it in regions with a history of ice forming along the eaves, extending from the eave edge to a point at least 24 inches inside the exterior wall line, measured along the roof slope. Because that 24-inch rule is a horizontal measurement projected onto a sloped roof, steeper pitches need a longer physical strip of membrane to satisfy it than shallow ones. Valleys get full-length coverage regardless of pitch because they concentrate water flow from two roof planes.
How to use it
- Measure the horizontal distance from the exterior wall face to the roof edge (the overhang plus wall thickness).
- Add 24 inches to that distance to get the required horizontal inside-wall coverage per IRC R905.1.2.
- Convert that horizontal distance to a sloped distance using the roof's pitch multiplier.
- Divide the sloped distance by the roll width to find how many courses are needed up from the eave.
- Multiply the number of courses by the roll width and the total eave length to get the eave area.
- Add valley coverage, measured full-width along the entire valley length.
- Add a waste factor, then divide the total area by the roll's coverage to get rolls needed.
The formula
Required slope coverage
slope coverage = (overhang + 24 in) × √(rise² + 12²) / 12
Projects the 24-inch horizontal code minimum onto the sloped roof surface.
Courses needed
courses = ceil(slope coverage / roll width)
Each course overlaps the one below it; rounding up ensures the code minimum is met.
Rolls needed
rolls = ceil((eave area + valley area) × (1 + waste) / roll coverage)
Roll coverage is typically 200 sq ft for a 36 in × 66.7 ft roll.
Worked example
A house with a 100 ft eave length, 18 in overhang, 6:12 pitch, 36 in rolls, 20 ft of valley, 10% waste.
- Horizontal coverage: 18 + 24 = 42 in
- Pitch factor: √(6² + 12²) ÷ 12 = 1.118
- Slope coverage: 42 × 1.118 = 47 in
- Courses: ceil(47 ÷ 36) = 2
- Eave area: 100 × (2 × 36 ÷ 12) = 600 sq ft
- Valley area: 20 × 3 = 60 sq ft
- Total with waste: (600 + 60) × 1.10 = 726 sq ft
- Rolls: ceil(726 ÷ 200) = 4
Install two courses of 36 in ice and water shield along the eaves, cover the valleys, and buy 4 rolls.
Reference chart
| Pitch | Slope coverage needed | Courses needed |
|---|---|---|
| 2:12 | 42.6 in | 2 |
| 4:12 | 44.3 in | 2 |
| 6:12 | 47 in | 2 |
| 8:12 | 50.5 in | 2 |
| 10:12 | 54.7 in | 2 |
| 12:12 | 59.4 in | 2 |
Typical ice and water shield roll sizes
| Roll width | Roll length | Coverage |
|---|---|---|
| 36 in | 66.7 ft | 200 sq ft |
| 36 in | 33.5 ft | 100 sq ft |
| 48 in | 33.5 ft | 134 sq ft |
Common mistakes
- Measuring the 24 inch requirement along the horizontal plan instead of projecting it up the actual roof slope.
- Skipping valleys because the eave is already covered — valleys need their own full-length run of membrane.
- Forgetting that larger overhangs push the required course count higher, not lower.
- Overlapping courses with too little lap, reducing the seal at the critical ice-dam zone.
- Using standard underlayment instead of self-adhered membrane in the ice dam zone, which does not seal around nail penetrations.
Assumptions and limits
- IRC R905.1.2's 24 inch inside-the-wall requirement is used as the governing minimum.
- Courses are measured in whole increments of roll width, rounded up for full code compliance.
- Valleys are assumed to need full standard roll width coverage along their entire length.
- Local jurisdictions in severe ice-dam climates may require more than the IRC minimum.
Frequently asked questions
How far up the roof does ice and water shield need to go?
IRC R905.1.2 requires it to extend from the eave edge to at least 24 inches inside the exterior wall line, measured along the roof slope, in areas with a history of ice damming.
How many rolls of ice and water shield do I need?
Calculate the total area covered along the eaves and valleys, add about 10% waste for overlaps and cuts, then divide by the roll's coverage — typically 200 sq ft for a standard 36 in roll.
Does roof pitch change how much ice and water shield I need?
Yes. The 24 inch code minimum is a horizontal measurement, so steeper roofs need a physically longer strip of membrane up the slope to satisfy the same horizontal distance.
Do valleys need ice and water shield even without ice dam risk?
Most roofers install it in all valleys regardless of climate zone, because valleys concentrate water flow from two planes and are a common leak point even without ice damming.
Can I use two courses instead of one wider course?
Yes, overlapping two standard-width courses is the normal way to reach coverage beyond a single roll's width, provided each course laps the one below by the manufacturer's minimum overlap.
What areas besides eaves and valleys need ice and water shield?
Many roofers also apply it around roof penetrations, at chimneys, in roof-to-wall transitions, and at low-slope sections under 4:12 pitch where standard underlayment alone is not sufficient.
Is ice and water shield required everywhere in the US?
No. IRC R905.1.2 ties the requirement to areas with a history of ice forming along the eaves; southern and mild-climate jurisdictions often do not require it, though it is still common good practice.
What is the difference between ice and water shield and synthetic underlayment?
Ice and water shield is self-adhering and seals around nail penetrations, making it waterproof under ice dams. Synthetic underlayment is water-resistant but not adhered, and relies on shingles above it to shed water, not seal against standing ice.
Last updated 2026-10-08. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.