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 <strong>from the eave edge up 24–36 inches past the interior wall line</strong>; 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.