An ice dam forms when heat from the house melts snow on the roof, the meltwater runs down to the cold overhang and refreezes there, and the ice backs water up under the shingles. A self-adhering membrane, usually called ice and water shield, seals around the nails and keeps that water from entering the house. It goes along the eaves and in valleys, where leaks are most likely. This calculator estimates the area and the number of rolls.
How to use the ice and water shield calculator
- Enter the total eave length — the sum of all the roof edges that need protection.
- Enter the eave overhang and the roof pitch.
- Keep the 24-inch distance inside the wall, or change it if your code or the membrane maker requires something else.
- Add valley length and strip width if you are lining valleys.
- Enter the coverage of one roll and a percentage for laps and waste.
Ice and water shield formula
Eave strip width = (overhang + distance inside the wall) × pitch factor
Eave area = eave length × strip width
Valley area = valley length × valley strip width
Rolls = ⌈ (eave + valley) × (1 + extra) ÷ roll coverage ⌉
The overhang and the 24 inches are horizontal distances, but the membrane lies on the sloped roof deck, so the strip has to be longer. A 6/12 roof has a pitch factor of 1.1180, so a 36 inch horizontal run needs 40.2 inches of material.
Worked example: 40 ft of eave, 12 in overhang, 6/12 pitch
- Horizontal width to cover: 12 + 24 = 36 in.
- Along the slope: 36 × 1.1180 = 40.2 in.
- Area: 40 ft × 40.2 in ÷ 12 = 134 sq ft.
- One roll of about 200 sq ft covers it: 1 roll. With 10% for laps, area is 148 sq ft, which is still one roll.
Eave strip width for a 12 in overhang and 24 in inside the wall
| Roof pitch | Pitch factor | Strip width along the slope (in) | Area per 100 ft of eave (sq ft) |
|---|---|---|---|
| 3/12 | 1.0308 | 37.1 | 309 |
| 4/12 | 1.0541 | 37.9 | 316 |
| 6/12 | 1.1180 | 40.2 | 335 |
| 8/12 | 1.2019 | 43.3 | 361 |
| 10/12 | 1.3017 | 46.9 | 391 |
| 12/12 | 1.4142 | 50.9 | 424 |
Installation notes
- Clean, dry deck. The membrane needs a clean, dry roof deck; primers may be required in cold weather.
- Start at the eave. Run the first course along the eave, then lap each higher course over the one below it so that water runs over the joints, not into them.
- Overlap. Side laps are typically a few inches and end laps larger; follow the roll’s instructions.
- Gutters and drip edge. The membrane usually goes under the drip edge at the eave, and over it at the rake. Check the manufacturer’s details.
- Ventilation. Insulation and attic ventilation do more than any membrane to prevent ice dams.
When is it required?
The IRC requires an ice barrier in locations where there has been a history of ice forming along the eaves causing a backup of water. Local codes decide the exact requirement, so ask your building department. Even where it is not required, many roofers add it at the eaves and valleys on low-slope roofs.
The rest of the roof is covered by the Roofing Calculator and the Roof Sheathing Calculator. For the length of the slope see the Rafter Length Calculator, for pitch factors the Roof Pitch Calculator, and for metal roofs the Metal Roofing Material Calculator.
Frequently asked questions
How far up the roof does ice and water shield go?
Where an ice barrier is required, the International Residential Code (R905.1.2) calls for it to extend from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line of the building. Check your local code for the details.
Why does the pitch matter?
The 24 inches is measured horizontally from the wall. On a sloped roof the membrane has to run farther along the surface, so the strip width is the horizontal distance times the pitch factor √(1 + (rise ÷ 12)²).
Do valleys need ice and water shield?
Many roofers line valleys with a self-adhering membrane because they carry heavy water flow. The valley strip is commonly 36 inches wide (one full roll width), but follow the shingle manufacturer’s instructions.
How many rolls do I need?
Divide the total area, including your extra percentage, by the coverage printed on the roll. Roll sizes vary by brand; enter the actual coverage.
What about penetrations and sidewalls?
Chimneys, skylights, vents and wall junctions are normally flashed with additional strips. This calculator covers the eaves and valleys; add other areas by hand.