Enter eave length, overhang, roof pitch, and valley length to find rolls of ice and water shield membrane needed.
How to Use the Ice and Water Shield Calculator
Enter the eave length that needs protection, the overhang and pitch that determine how far up the slope the membrane must run, and any valley length. The roll count updates as soon as a field changes.
Ice dams form at the roof edge when heat escaping through the attic melts snow higher up the roof, which then refreezes right at the cold overhang and backs water up under the shingles. Ice and water shield is the self-adhering membrane that stops that backed-up water from reaching the deck, but only if it runs far enough up the slope to sit past where the dam actually forms.
Coverage Width, Not Just Length
Most US building codes, following the IRC's R905.1.2 language, require the membrane to extend from the eave edge to a point at least 24 inches inside the building's exterior wall line, measured horizontally. Because the membrane runs up a sloped surface rather than flat ground, that horizontal distance has to be converted to an actual slope length before it turns into a roll count:
coverage width (slope) = (overhang + 24") × √(1 + (pitch ÷ 12)²)
eave area = eave length × coverage widthValleys Get Full-Width Coverage
Valleys concentrate runoff from two roof planes into one channel, which makes them just as vulnerable to ice damming as eaves, so most installers run a full roll width centered directly on the valley line rather than calculating a partial width. This calculator does the same, multiplying valley length by the roll width entered rather than the eave's coverage-width formula.
A standard roll covers about 200 square feet, roughly 3 feet by 66.7 feet — check the actual spec on the product you're buying, since some run shorter.