Ice and Water Shield Calculator

The calculator works in four steps. First it finds the slope distance the membrane must cover at the eave: the overhang plus 6 in of wall plus the distance past the inside of the wall, multiplied by a pitch factor. With the defaults that is 12 + 6 + 24 = 42 in of horizontal distance, which is 47.0 in along a 6/12 roof. Second, it counts the courses. One 36 in roll covers 33 in of slope once the 3 in headlap is allowed for, so 47.0 in needs 2 courses along the eave.

Third, it adds up the area. Two courses along 120 ft of eave is 2 x 3 ft x 120 ft = 720 sq ft. A 30 ft valley with a 3 ft strip is 90 sq ft, and 2 penetrations at 25 sq ft each is 50 sq ft, for 860 sq ft in total. Fourth, it adds a 10% waste allowance and divides by the roll size: 860 x 1.1 = 946 sq ft, and 946 / 200 = 4.73, which rounds up to 5 rolls. At $95 a roll that is $475, and installation at $45 a roll adds $225, for $700.

Rolls come in whole units, so small changes can add a whole roll or none at all. The same house without the valley and the penetrations covers 720 sq ft and needs 4 rolls, or $560. A 225 sq ft roll also comes to 5 rolls here, but at $115 a roll the total is $800, so a bigger roll is not automatically the cheaper buy. Compare the price per sq ft of the roll you can actually get.

The model code is section R905.1.2 of the International Residential Code, which covers ice barriers. In both the 2021 and 2024 editions it applies in areas where there has been a history of ice forming along the eaves and causing a backup of water, as designated in Table R301.2. Where it applies, an ice barrier goes under asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. It can be at least two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment.

The barrier runs from the lowest edges of all roof surfaces to a point at least 24 in inside the exterior wall line of the building. On roofs with a slope of 8 units vertical in 12 or steeper, it must also reach at least 36 in measured along the roof slope from the eave edge. Detached accessory structures that do not contain conditioned floor area are excepted. The calculator never falls short of the 36 in rule: with the smallest overhang and distance past the wall it allows, the slope distance on an 8/12 roof is already about 36.1 in.

Whether the requirement applies to your house is a local question. States and towns adopt different editions of the code and may amend this section, so the local building department decides whether you need an ice barrier and where it must stop. Ask them before you order, and enter the distance past the wall that they require. The calculator measures that distance from the inside face of the wall and adds a 6 in wall thickness, which errs on the side of covering a little more than the bare 24 in.

The code distance is measured across the house in plan, but the membrane lies on the slope, and a steeper slope is longer for the same horizontal distance. At the default 42 in of horizontal distance, a 6/12 roof needs 47.0 in of membrane, a 12/12 roof needs 59.4 in and an 18/12 roof needs 75.7 in. The first two still fit in 2 courses, but 75.7 in needs 3, which takes the default job from 5 rolls ($700) to 7 rolls ($980). With the default overhang, 16/12 is the first whole pitch that gives 3 courses.

Overhang works the same way, and it is the one most often forgotten. A roof with no overhang at 6/12 needs only 33.5 in, a single course of 1 roll width, and with the other defaults it comes to 500 sq ft, 3 rolls and $420. Going the other way, a 24 in overhang makes the slope 60.4 in and stays at 2 courses, while a 36 in overhang makes it 73.8 in and needs 3 courses, 1,220 sq ft, 7 rolls and $980.

The extremes add up quickly. A 12/12 roof with a 36 in overhang and the membrane carried 36 in past the wall comes to a slope of 110.3 in, which needs 4 courses. For 120 ft of eave with the default valley and penetrations that is 1,580 sq ft, 9 rolls and $1,260. If you are close to a course boundary, measure the roof rather than trusting the plan dimensions, because the code distance is a minimum.

Eaves are where ice dams form, but they are not the only place a roof leaks. Many installers also line valleys and seal around chimneys, skylights, vents and pipes with the same membrane, which is why the calculator includes a 3 ft strip for valleys and rakes and 25 sq ft for each penetration. Treat these as optional scope: set valleys, rakes or penetrations to 0 if your roofer is using another product there. Adding a 100 ft valley to the default job takes it from 860 sq ft to 1,070 sq ft and from 5 rolls to 6.

The membrane protects the roof deck from water that backs up behind ice. It does not stop ice dams from forming. The University of Minnesota Extension explains that ice dams come from heat loss from the house, snow cover and outside temperatures acting together, and it says that ice dams can be prevented by controlling the heat loss from the home. The U.S. Department of Energy's Building Science Education resource recommends sealing the eaves with a peel-and-stick bituminous membrane in Climate Zone 5 and higher, alongside air sealing, attic insulation and ventilation that reduce heat transfer to the roof deck.

In practice that means ice and water shield is cheap insurance and not a cure. If the attic leaks warm air, the ice will come back every winter, and the membrane is only the last line of defense when it does. Pair the membrane with air sealing and enough insulation, and have a roofer or weatherization contractor look at the attic.