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A standard roll is 36 inches wide by 66.7 feet — 200 square feet, or two squares. Code generally requires the barrier to run from the eave edge to at least 24 inches inside the exterior wall line, measured along the slope, so a low pitch with a deep overhang needs two courses.
Estimate rolls of ice and water barrier for eaves, valleys, and penetrations. Code-compliant widths built in.
Ice and water shield is a self-adhering membrane that seals around fasteners and bonds directly to the deck. It goes where water is most likely to back up: the eaves in freezing climates, every valley, and around every penetration.
| Pitch | Slope distance to reach 24 in inside the wall | Courses of 36 in membrane |
|---|---|---|
| 3/12 | 41.2 in | 2 |
| 4/12 | 42.2 in | 2 |
| 6/12 | 44.7 in | 2 |
| 8/12 | 48.1 in | 2 |
| 12/12 | 56.6 in | 2 |
Assumes a 16-inch overhang and 2 inches of lap between courses. Code generally requires the barrier to extend from the eave edge to at least 24 inches inside the exterior wall line, measured along the slope — so a low pitch with a deep overhang needs two courses.
| Valley length | Rolls at 36 in x 66.7 ft |
|---|---|
| 12 ft | 0.2 |
| 20 ft | 0.3 |
| 32 ft | 0.5 |
| 48 ft | 0.7 |
| 64 ft | 1 |
Run it the full length of every valley, centred on the valley line, before the field underlayment goes on. Around penetrations, a piece large enough to lap under the flange on all sides.
Not across the whole deck in a mild climate. It is not breathable, and over a whole roof it can trap moisture in the deck. Use it where water backs up and normal underlayment everywhere else — that is what each product is designed for.
From the eave edge to at least 24 inches inside the exterior wall line, measured along the slope. On a low pitch with a deep overhang that means two courses of 36-inch membrane, not one.
Yes, the full length of every valley, centred on the valley line and laid before the field underlayment. Valleys carry water from two planes and freeze first.
Under metal or tile, yes. Deck temperatures under those coverings run far higher than under asphalt, and a standard grade can soften and lose adhesion.
Not in a mild climate. It is not breathable and over a whole deck it can trap moisture. Use it where water backs up and normal underlayment everywhere else.
Choosing the rest of the underlayment? See the roof underlayment buying guide.
Eaves that dam every winter? The roof heat cable buying guide covers cable types and how much you need.
Choosing the membrane? See the best ice and water shield compared on grip, temperature range and roll size.
Gutters still freezing? The roof heat tape calculator sizes de-icing cable by EasyHeat's method.
Leaking from an ice dam right now? The ice dam removal cost calculator shows what a steam crew charges.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.