Ice and Water Shield: When You Actually Need It (and How Far Up the Roof)
Ice and water shield isn't a blanket requirement everywhere, and 'just run it the whole roof' wastes money on jobs where it isn't doing anything useful. What actually determines whether you need it, and how far up from the eave, is climate zone, roof pitch, and valley/penetration geometry -- not a flat rule of thumb.
What it's actually for
Ice and water shield is a self-adhering waterproof membrane installed under the roof covering. Its job is narrow and specific: stop water from an ice dam -- water that's backed up and sitting under shingles, not just running off them -- from getting into the deck. Normal underlayment sheds water moving downhill; it does nothing once water is sitting still or moving uphill under a dam, which is exactly what ice and water shield is built for.
When it's required vs. when it's just smart
Most cold-climate building codes (IRC-based, but confirm your local adopted version) require it in areas with a history of ice damming, typically from the eave edge to a point at least 24 inches inside the exterior wall line. In warmer climates without freeze-thaw cycles, code often doesn't require it at all -- but valleys, roof-to-wall intersections, skylight curbs, and low-slope sections (under about 4:12) are worth it regardless of climate, because those are wind-driven-rain risk points, not just ice-dam risk points. The distinction matters: code minimum protects against ice damming specifically; the valley/penetration case is about wind-driven rain and applies almost everywhere.
How far up the roof, in practice
The 24-inches-past-the-wall-line rule is a minimum, not a target. On a shallow-pitch roof, or in a climate with genuinely severe ice damming, that 24 inches can end up well short of where ice dams actually form -- the correct method is to calculate the distance based on roof pitch and the point where the interior wall's warm side meets the roof plane, not just measure 24 inches and stop. This is a case where following the code minimum literally, without checking whether it matches your actual roof geometry, is a common and expensive mistake -- expensive because it shows up as a leak two winters later, not on inspection day.
Common mistake worth flagging
Installers sometimes run ice and water shield over the entire roof deck 'to be safe.' Beyond a certain point this isn't extra safety, it's a vapor-barrier problem: ice and water shield doesn't breathe, so a full-deck application on a roof assembly that isn't designed for it can trap moisture from the underside, especially over unconditioned attic space. If full coverage seems warranted, it needs to be paired with the right ventilation strategy, not applied as a blanket upgrade.