Where the membrane goes
Ice & water shield is a self-adhering rubberized membrane installed along eaves, in valleys and around penetrations — anywhere ice dams force meltwater up under shingles. Cold climates require it from the eave edge up 24–36 inches past the interior wall line; the calculator sizes rolls (225 sq ft each) from your eave perimeter × coverage width.
Valleys need it even in warm states
Valleys carry water from two roof planes and leak first in any climate. Standard practice: one full roll width down every valley, plus the eave courses in snow country. It goes on tight to clean, dry decking — this membrane is unforgiving about repositioning, so measure the valleys before peeling anything.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- National Roofing Contractors Association — installation standards for steep- and low-slope roof systems.
- FEMA Building Science — wind and snow load guidance for residential roofs.
- American Society of Civil Engineers — ASCE 7, the source of the ground snow load maps used in roof design.
Figures, formulas and reference tables last verified: 19 September 2026.