Ice and water shield calculator: how much ice and water shield do I need at the eaves?
3 rolls
of 75 ft · 2 courses of 36 in for 40 1/4 in up the slope, 160 linear ft · 36 in level, overhang + 24 in · for 80 ft of eaves at 6/12
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
3 rolls
of 75 ft · 2 courses of 36 in for 40 1/4 in up the slope, 160 linear ft · 36 in level, overhang + 24 in
Source: 2021 International Residential Code, R905.1.2 Ice barriers (2021 Seattle Residential Code, chapter 9), read October 21, 2026. Data sheet (PDF)
80 feet of eaves with a 12 inch overhang on a 6/12 roof take 3 rolls of 75 ft Grace Ice & Water Shield: IRC 2021 R905.1.2 runs the barrier to 24 inches inside the wall line, 40 1/4 inches up the slope, two courses.
A worked example
113 ft of eaves with an 18 inch overhang on a 9/12 roof, in rolls of 66.6 ft needs 4 rolls. The barrier reaches 42 inches measured level, 52 1/2 up a 9/12 slope, past the 36 inch minimum for 8/12 and steeper: two courses, 226 linear feet, which is 3.41 rolls with 6 inch end laps, so 4.
How far up the roof does ice and water shield go?
To at least 24 inches inside the exterior wall line, from the lowest edge of the roof, by IRC 2021 R905.1.2; on a roof of 8/12 or steeper, also at least 36 inches up the slope. With a 12 inch overhang that is 36 inches measured level and 40 1/4 inches up a 6/12 roof, so one 36 inch course is not enough.
How many square feet are in a roll of ice and water shield?
225 square feet in the 75 ft roll of Grace Ice & Water Shield, 200 in the 66.6 ft roll and 108 in the 36 ft roll, all 36 inches wide, by its data sheet. Side laps of 3.5 inches leave 32.5 inches of new coverage for each extra course.
What else it is called
Ice and water shield is the Grace product name that became the generic one; the code calls it an ice barrier, and it is also sold as self-adhered underlayment or ice and water barrier. It is searched as an ice and water shield calculator or how much ice and water shield do I need.
Ice barrier width by overhang and pitch
| Overhang | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| 0 in | 25 1/2 in, 1 course | 27 in, 1 course | 36 in, 1 course | 36 in, 1 course | 36 in, 1 course |
| 12 in | 38 in, 2 courses | 40 1/4 in, 2 courses | 43 1/2 in, 2 courses | 47 in, 2 courses | 51 in, 2 courses |
| 18 in | 44 1/2 in, 2 courses | 47 in, 2 courses | 50 1/2 in, 2 courses | 54 3/4 in, 2 courses | 59 1/2 in, 2 courses |
| 24 in | 50 3/4 in, 2 courses | 53 3/4 in, 2 courses | 57 3/4 in, 2 courses | 62 1/2 in, 2 courses | 68 in, 2 courses |
Width of ice barrier up the slope to reach 24 in inside the exterior wall line, IRC 2021 R905.1.2, and at least 36 in at 8/12 and steeper; courses of 36 in membrane with the 3 1/2 in side lap of the Grace Ice & Water Shield data sheet.
Where the numbers come from
- How far up: 2021 International Residential Code, Section R905.1.2: the ice barrier extends «from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line of the building», and on roofs of 8 in 12 or steeper it is also applied «not less than 36 inches (914 mm) measured along the roof slope from the eave edge». Read in the 2021 Seattle Residential Code and checked against the 2021 IRC as adopted in New Jersey, which changes where a barrier is required but not how far it runs. Read October 21, 2026. Read the code chapter (PDF)
- Rolls and laps: Grace Ice & Water Shield product data sheet (W. R. Grace, 2014): rolls 36 inches wide, 75 ft (225 sq ft), 66.6 ft (200 sq ft) and 36 ft (108 sq ft); side laps at least 3.5 inches and end laps at least 6 inches, so a second course adds 32.5 inches up the slope. Read October 21, 2026. Data sheet (PDF)
- The rest is geometry: level width = overhang + 24 inches; up the slope = level width × √(1 + (rise ÷ 12)²), the roof pitch factor; rolls = (courses × eave length − 0.5 ft) ÷ (roll length − 0.5 ft), rounded up.
Whether a roof needs an ice barrier at all is set by Table R301.2 of each jurisdiction; New Jersey, for one, ties it to a January average of 25°F or less. This page sizes the barrier at the eaves; valleys, rakes and penetrations are not counted.
By Javier Miralles · Last updated . How this site is made.
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