Ice and Water Shield Calculator
Ice and water shield calculator: eave length × band width ÷ 200 sq ft per roll. A 40 ft eave with a 6 ft band is 240 sq ft, or 2 rolls ($200).
An ice and water shield calculator multiplies the length of the edge you are protecting by the width of the band, then divides by 200 sq ft per roll and rounds up. A 40 ft eave with a 6 ft band is 240 sq ft, which is 1.2 rolls, so you buy 2. At the default $100 per roll that is $200.
The formula in plain words
Treat the membrane as a strip of tape along the lowest edge of the roof. The strip is as long as the eave and as wide as your local rules or the manufacturer calls for.
Multiply length by width and you have the area of tape. Each roll covers 200 sq ft, so divide by 200. You cannot buy 0.2 of a roll, so any leftover fraction rounds up to another full roll.
Worked examples
Example 1, the default: 40 ft of eave, 6 ft band, $100 per roll.
- Area: 40 × 6 = 240 sq ft
- Rolls: 240 ÷ 200 = 1.2, rounded up to 2
- Cost: 2 × $100 = $200
Example 2, a house with a 100 ft eave line on a 6/12 roof. The overhang is 12 inches and the code in this example asks for membrane 24 inches inside the wall.
- Horizontal run: 12 + 24 = 36 inches
- Along the slope: 36 × 1.118 = 40.25 inches, call it 3.5 ft
- Area: 100 × 3.5 = 350 sq ft
- Rolls: 350 ÷ 200 = 1.75, rounded up to 2
The horizontal run has to be stretched by the pitch multiplier, because the membrane lies on the slope. This is the step people skip.
Rolls by eave length and band width
| Eave length | Band width | Area | Rolls |
|---|---|---|---|
| 40 ft | 3 ft | 120 sq ft | 1 |
| 60 ft | 3 ft | 180 sq ft | 1 |
| 120 ft | 3 ft | 360 sq ft | 2 |
| 40 ft | 6 ft | 240 sq ft | 2 |
| 100 ft | 6 ft | 600 sq ft | 3 |
How to measure
Measure each eave edge and add them together. Gable ends (rakes) are measured separately if you are covering them, and valleys are measured on their own, because their width is different.
For the band width, use the figure that your local code or the shingle and membrane instructions give for your climate. Work it out as a horizontal distance first, then multiply by the pitch multiplier from the roof pitch calculator to get the length along the slope.
Common mistakes
- Using the horizontal distance as the width. On a steep roof this undercounts the membrane noticeably.
- Forgetting laps. A 6 ft band takes two 3 ft courses, and the overlap between them uses up a few inches of every roll.
- Skipping valleys. They are measured separately and usually need their own rolls.
- Counting the whole roof. Only the edge zones get membrane. The rest gets ordinary underlayment.
- Ordering exactly what the math says on a tight job. A roll that comes up 5 sq ft short is a second trip.
Why the eaves get special treatment
Ice dams form when heat from the house melts snow on the roof. The meltwater runs down to the cold overhang and refreezes there, building a ridge of ice. Water behind that ridge backs up under the shingles. A self-adhering membrane seals around the nails and keeps that water out of the deck.
Insulation and attic ventilation reduce how much heat reaches the roof, and the membrane is the backup. Follow the manufacturer’s instructions for deck temperature and surface prep, because the adhesive works only on a clean, dry deck within the temperatures stated on the roll.
Ordering the right product
Self-adhering membranes come in different widths and lengths, and some are made for high heat under metal roofs. Check that the product is rated for the roof covering above it. The roll size matters for the math, so if your roll covers something other than 200 sq ft, divide your area by the actual coverage printed on the label.
Related calculators
The rest of the roof needs roofing underlayment, and the roofing felt calculator handles the older felt option. The roofing shingle calculator finds the bundles for the top layer, and the roof area calculator gives the total surface.
Frequently asked questions
How much roof does one roll of ice and water shield cover?
The calculator uses 200 sq ft per roll, a 2 square roll. Rolls are commonly 36 inches wide, so 200 sq ft is about 67 ft of length. If you need laps between courses, you lose some coverage on each roll, so round up when in doubt.
What do the length and width fields mean here?
Length is the run of eave or valley you are covering, in feet. Width is the band you want covered, measured up the slope, also in feet. A 40 ft eave with a 6 ft band is 240 sq ft, which is 1.2 rolls and rounds up to 2.
Why is there no waste toggle for this calculator?
Rolls are sold whole, so the rounding up already leaves a cushion on most small jobs. A 240 sq ft area uses 1.2 rolls and you buy 2. If your layout has many laps or valley cuts, add one more roll by hand.
How far up the roof does ice and water shield need to extend?
Many codes ask for the membrane to run from the eave edge to at least 24 inches inside the exterior wall line, but local rules differ and some areas require none. A 12 inch overhang plus 24 inches is 36 inches horizontal, about 40 inches along a 6/12 slope. Check your local code before ordering.
Do valleys need the same membrane as eaves?
Valleys are a common spot for leaks, and many roofers line them with the same membrane. Measure each valley's length and enter a band width from the product instructions, often about 3 ft total. Run valleys as a separate calculation and add the rolls to the eaves.
Estimates only. Verify quantities with your supplier before purchasing.