Ice and Water Shield Calculator
Calculate how many rolls of ice and water shield underlayment you need for roof eaves, valleys, and penetrations.
Covers coverage requirements per building code for cold climates.
What Is Ice and Water Shield? Ice and water shield (I&W) is a self-adhering, waterproof roofing underlayment. It seals around nails and fasteners, preventing water from leaking even if shingles are damaged or ice dams form. Required by most building codes in cold climates — typically at eaves, valleys, and around penetrations. Common brand: Grace Ice & Water Shield® (invented by Grace Construction Products, United States, 1978).
Where It’s Required Most codes (IBC, IRC) require I&W at eaves extending from the roof edge to at least 24 inches inside the heated wall line. In climates with severe ice dams: extend I&W at least 36"–48" up from eave, sometimes 6 feet. Required at all valleys (the V-shaped channel where two roof slopes meet). Recommended (sometimes required) around chimneys, skylights, and pipe penetrations. Optional: some roofers use I&W on the entire roof for maximum protection.
Standard Roll Sizes Most I&W products: 36 inches wide × 65 feet long = 195 sq ft per roll. Some products: 36" × 75 ft = 225 sq ft per roll. Overlap: typically 2"–4" overlap on horizontal joints; 6" overlap at end joints. Effective coverage is slightly less than total sq ft due to overlaps.
Code Requirements by Climate Zone Climate Zone 5 and higher (most of Canada, northern US, mountain states): eave coverage required. IRC (International Residential Code): I&W at eave must extend from edge to 24" inside the exterior wall. ASCE 7 ice load maps and local amendments may specify additional coverage. Always verify with local building department — requirements vary by jurisdiction.
Ice Dam Formation Ice dams form when heat escapes through the roof, melts snow at the upper roof, and the meltwater refreezes at the cold eave. Water backs up behind the ice dam and can penetrate under shingles. I&W barrier prevents this water from entering the structure even when water runs under shingles. Best defense: air sealing and insulation to prevent heat loss — I&W is the second line of defense.
Valley Coverage I&W in valleys should extend at least 18" on each side of the valley centerline. For a 36"-wide roll centered in a valley: provides 18" coverage per side — exactly what code requires. Longer valleys need multiple strips overlapping by at least 6" at joints.