How this calculator works
The membrane has to run from the lower edge of the roof to a set distance inside the warm wall, measured along the slope. The horizontal distance is the overhang, plus the wall thickness, plus the 24 inches (or 36) inside the wall. Along the roof it is longer by the slope factor, which is √(1 + (pitch ÷ 12)²). Multiply by the eave length. Valleys take a strip about 3 ft wide, and each penetration about 10 ft².
Worked example: 80 ft of eaves with a 12 inch overhang, a 6 inch wall and 24 inches inside, on a 6/12 roof, needs a strip 3.91 ft up the slope: 313 ft². With 2 penetrations and 10% extra that is 366 ft², which is 2 rolls of 200 ft². On a 12/12 roof the width is 4.95 ft, and 30 ft of valley adds 90 ft².
Rolls for the eaves by roof pitch (80 ft of eaves, 12 in overhang, 6 in wall, 24 in inside)
Width up the slope, square feet and rolls.
| Pitch | Width up the slope | Square feet | Rolls |
|---|---|---|---|
| 3/12 | 3.61 ft | 289 | 2 |
| 4/12 | 3.69 ft | 295 | 2 |
| 6/12 | 3.91 ft | 313 | 2 |
| 8/12 | 4.21 ft | 337 | 2 |
| 10/12 | 4.56 ft | 364 | 3 |
| 12/12 | 4.95 ft | 396 | 3 |
Tips before you buy
- Check the code: some areas want the membrane 24 inches inside the warm wall, others a certain distance from the eaves, and some need it on low pitches everywhere.
- Valleys, around chimneys and skylights, and where roofs meet walls are the other places to use it.
- Roll it onto a clean, dry, smooth deck, starting at the eaves, with laps of 3 to 6 inches as the roll says.
- Work in the right temperature range. The membrane is hard to stick in the cold and slippery when hot.
- Lay the membrane over the drip edge at the eaves, and fit the drip edge over the underlayment at the rakes, as the shingle maker shows.
Tools & materials for this project
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Frequently asked questions
How much ice and water shield do I need?
For 80 ft of eaves on a 6/12 roof with a 12 inch overhang, about 366 ft² with 10% extra, which is 2 rolls of 200 ft². Valleys and penetrations are extra.
How far up the roof should ice and water shield go?
Most codes say at least 24 inches inside the exterior wall line, measured along the roof, in cold climates. Some areas ask for 36 inches. Check the local code.
How big is a roll of ice and water shield?
Most rolls are 36 inches wide and about 67 feet long, which is 200 square feet.
Do valleys need ice and water shield?
Yes, usually: a strip about 3 ft wide along the valley. 30 ft of valley adds 90 ft².
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.