Understanding your result
The headline is the number of rolls. The stats show the distance the membrane must reach up the slope, the number of courses that takes, and the total membrane area with waste. The breakdown starts with the three horizontal distances the code adds up, converts them to a slope distance with the pitch factor, and then counts courses, eave footage, valleys and penetrations.
The number to remember is the courses at the eave. Two courses of a 36-inch roll is the normal answer on a house with an overhang, and it decides the whole order. A long eave with two courses needs twice the roll footage of the eave, which is why the underlayment calculator note says the membrane replaces the underlayment in that strip rather than adding to it.
The valley line, when you enter one, is the same length the valley flashing calculator uses, with the same foot of run-out at each eave.
How we calculate this
The one step that is not obvious is the conversion to the slope. The code’s 24 inches is a horizontal measurement from the inside face of the wall to the point above which the membrane must reach, because what it is protecting is the heated part of the house behind the wall. The roofer, though, is unrolling membrane up a sloped deck, and a horizontal distance of 42 inches is 47 inches of deck on a 6/12 roof and 59 inches on a 12/12. The steeper the roof, the more courses the same code sentence demands. The pitch factor table gives the multiplier for every pitch, and the pitch calculator converts degrees.
Courses overlap by the lap you set. Most manufacturers require at least 3½ inches; six is common practice and the default here, so the second course adds 30 inches of reach, not 36.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Distance inside the wall line | 24 in | IRC R905.1.2 (ice barrier to at least 24 in inside the exterior wall line) |
| Overhang | 12 in | Common US eave overhang; measure yours |
| Wall thickness | 6 in | A 2×4 wall with sheathing and siding; use 8 in for 2×6 |
| Roll size | 36 in × 66.7 ft (200 sq ft) | Standard self-adhering membrane roll (GAF WeatherWatch, IKO StormShield and similar); some rolls are 75 ft |
| Overlap between courses | 6 in | Manufacturers require 3½ in minimum side laps; 6 in is common practice |
| Valley strip | Full roll width, centred | IRC R905.2.8.2 valley lining and manufacturers’ instructions |
| Membrane per penetration | 20 sq ft | A full-width piece about 3 ft tall wrapped above and beside a chimney or skylight |
| Waste | 10% | Estimating practice |
Assumptions last reviewed October 7, 2026.
The calculator does not decide whether your area requires an ice barrier (that is the local amendment to R905.1.2), does not model covering the whole deck, and does not count the membrane some manufacturers want under metal roofing or around skylights beyond the 20 square feet allowance. The guide to where ice and water shield is required walks through the code language, the climate zones that apply it and the low-slope cases where the membrane goes further.
Two worked examples
A standard eave on a 6/12 roof
Eighty feet of eaves, 12-inch overhang, 6-inch wall, 24 inches inside the wall, 6/12 pitch, 36 in × 66.7 ft rolls with a 6-inch lap, no valleys, 10% waste.
- Horizontal distance: 12 + 6 + 24 = 42 in
- Up the slope: 42 × 1.118 = 47 in
- Courses: one 36-in course is short, so 1 + round up ((47 − 36) ÷ 30) = 2 courses, covering 66 in
- Eave membrane: 80 × 2 × 3 ft = 480 sq ft
- With waste: 528 sq ft; rolls: 528 ÷ 200 = 2.64, rounded up to 3 rolls
Three rolls, with most of one spare for the pipe boots and a skylight. The second course is what the pitch costs; on a flat deck 42 inches would have fit in two courses just the same, so the slope matters more as the pitch rises.
A low-slope roof with wide overhangs, valleys and two penetrations
A 4/12 roof with 120 feet of eaves, 18-inch overhangs, 36 feet of valleys and two penetrations.
- Horizontal distance: 18 + 6 + 24 = 48 in
- Up the slope: 48 × 1.054 = 50.6 in, so 2 courses again (66 in covered)
- Eave membrane: 120 × 2 × 3 = 720 sq ft
- Valleys: 36 ft × 3 ft = 108 sq ft; penetrations: 2 × 20 = 40 sq ft
- Total with 10% waste: (720 + 108 + 40) × 1.10 = 955 sq ft
- Rolls: 955 ÷ 200 = 4.77, rounded up to 5 rolls
The wider overhang nearly pushed the eave to a third course. On a 4/12 roof the manufacturer may also recommend 36 inches inside the wall rather than 24; change that field and the calculator shows three courses and six rolls.
Where to find your inputs
Eave length. Every eave of heated space. Porch roofs over unheated space and detached garages are exempt by code, though many roofers include them anyway.
Overhang. Horizontal, from the outside face of the wall to the drip edge, measured under the eave.
Wall thickness. From the outside of the sheathing to the inside of the drywall: about 6 inches for a 2×4 wall with siding, 8 inches for 2×6, more for brick veneer. Remember the code measures from the inside of the wall.
Pitch. From a level and a ruler on a rake board or rafter, or from the pitch calculator. The pitch decides the slope distance and therefore the number of courses.
Valley length. Along the valley line, from the ridge to the eave, for each valley; the calculator adds a foot of run-out each.
Common mistakes
- Measuring the 24 inches up the slope. The code measures horizontally inside the wall. On a steep roof the slope distance is much longer, and one course is short.
- Starting from the wall instead of the drip edge. The membrane begins at the lowest edge of the roof, so the overhang is inside the distance. Twelve inches of overhang is a third of the horizontal requirement.
- Under the drip edge at the eave. The membrane goes over the eave drip edge, down to its outer edge, so water running off the membrane lands on the metal, not behind it.
- Skipping the valleys. Valleys hold snow and channel meltwater; they need a full-width strip whether or not the eaves do.
- Leaving it uncovered. The membrane is not UV-stable. It must be covered by the underlayment and roofing within the exposure limit on the wrapper, usually 30 to 90 days.
- Applying it cold. Most membranes need the deck above about 40 °F (5 °C) to bond; below that, a primer or a warm day is required, or the laps lift.
Questions people ask
- Where is ice and water shield required by code?
- The International Residential Code (R905.1.2) requires an ice barrier where there is a history of ice forming along the eaves, running from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. Local amendments decide which areas count; most of the northern United States and all of Canada apply it. Detached buildings without heated space are exempt.
- How many courses of ice and water shield do I need at the eaves?
- Usually two of a 36-inch roll. With a 12-inch overhang and a 6-inch wall the horizontal distance to 24 inches inside the wall is 42 inches, and on a 6/12 pitch that is 47 inches up the slope. One 36-inch course falls short; two courses with a 6-inch lap cover 66 inches. Only a roof with no overhang on a shallow pitch gets away with one.
- How much does a roll of ice and water shield cover?
- A standard roll is 36 inches by 66.7 feet, 200 square feet or two squares; some brands run 75 feet (225 square feet). The calculator's default roof, 80 feet of eaves with two courses, uses 160 feet of roll, 480 square feet, which is three rolls with the 10 percent waste allowance.
- Does ice and water shield go under or over the drip edge?
- Over it at the eaves. The eave drip edge goes on the bare deck first, the membrane goes over the drip edge flange and down to its outer edge, and the underlayment above laps over the top of the membrane. At the rakes the order reverses: drip edge over the underlayment.
- Should I put ice and water shield in the valleys?
- Yes. The code requires a valley lining under closed valleys and the membrane is the usual choice; it also serves as the ice barrier the valley needs, since valleys collect snow. Centre a full-width strip on the valley line for its whole length plus a foot of run-out at the eave. Enter the valley length and the calculator adds it.
- Can ice and water shield cover the whole roof?
- It can, and on pitches below 4/12 or in severe climates roofers often do it, but a fully sealed deck does not dry inward, so the attic must be well ventilated, and the cost is several times that of synthetic underlayment. The membrane also needs a cover; it is not rated for permanent exposure.