Ice and Water Shield Calculator
Rolls of ice and water shield for your eaves, from their length, the pitch and how far up the roof it must reach.
Result
Enter distance inside the exterior wall line, roll width and roll length to see the result.
https://calcgallery.com/roofing/ice-water-shield-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Ice and Water Shield Calculator
- Add up the length of every eave the membrane will run along.
- Enter the pitch, the eave overhang measured level, and how far inside the exterior wall line your local code asks the barrier to reach. Add a minimum width along the slope only if your code sets one.
- Enter the roll's width and length and the laps from the product instructions.
From the wall line to the slope
width along the slope = (eave overhang + distance inside the wall) × √(1 + (rise ÷ 12)²)courses = 1, or 1 + (width − roll width) ÷ (roll width − side lap), rounded uprolls = courses × eave length ÷ (roll length − end lap), rounded upThe level distance from the eave edge to the point inside the wall is the overhang plus the inside distance. The slope factor turns that level distance into the length of membrane that lies on the roof. If your code also sets a minimum width along the slope, the page uses whichever is wider.
Checking the code
Whether an ice barrier is needed at all, how far it reaches and what products qualify are decided by the building code and the roofing manufacturer where the house is. They differ from place to place and between code editions, so check with your building department before you order.
With your numbers
- Width along the slope = (overhang + distance inside the wall) × √(1 + (rise ÷ 12)²)
- Rolls = courses × eave length ÷ (roll length − end lap), rounded up
Quick reference
Rolls round up; valleys and penetrations need their own pieces.
| Job | Rolls to buy |
|---|---|
| 84 ft of eave, 6 in 12, 24 in inside | 3 |
| 8 in 12 with a 36 in slope minimum | 3 |
| 60 ft of eave, 4 in 12, 6 in overhang | 1 |
| 120 ft of eave, 12 in 12 | 4 |
| 84 ft, 36 in inside the wall | 3 |
| 25 m of eave, 1 m by 20 m rolls | 3 |
Common questions about the Ice and Water Shield Calculator
- How many rolls of ice and water shield do I need?
- Three 36-inch by 75-foot rolls for 84 feet of eave at 6 in 12, with a 12-inch overhang and the strip running 24 inches inside the wall. Those 36 inches of level distance are 40.25 inches along the slope, so it takes two courses with a 3-inch side lap. Two courses of 84 feet are 168 feet of membrane, and 168 ÷ (75 feet less a 6-inch end lap) = 2.3 rounds up to 3 rolls.
- How far up the roof does ice and water shield go?
- Where codes require an ice barrier, they commonly measure it from the eave edge to a point a set distance inside the exterior wall line, so that the warm part of the roof above the living space is covered. The distance, and whether there is also a minimum measured up the slope, is set by the code where you build. This page takes both from you and states no requirement.
- Why is the strip wider on a steep roof?
- The distance inside the wall is measured level, but the membrane lies on the slope. On a 6 in 12 roof, every foot of level distance is 1.118 feet along the roof; on 12 in 12 it is 1.414 feet. The same 36 inches of overhang and inside distance is about 40 inches of membrane on the first roof and 51 inches on the second.
- How many courses will I need?
- If the strip is no wider than the roll, one. If it is wider, the next course laps the first by the side lap in the instructions, so each added course covers the roll width less that lap. A 40-inch strip with a 36-inch roll and a 3-inch lap needs two courses.
- Does this include valleys?
- No. Valleys, sidewalls, chimneys and skylights are often covered too, and each needs its own pieces. Add their lengths to the eave length, or buy an extra roll.