What underlayment does
Shingles shed water; they don't seal against it. Wind-driven rain, capillary creep at overlaps, and the occasional lifted tab all push moisture past the shingle layer as a matter of routine. Underlayment is the water-resistant sheet across the entire deck that catches that routine moisture and lets it drain harmlessly.
It also covers the roof during installation itself, which is why a weather pause mid-replacement isn't a crisis: the underlayment stage is built to hold watertight on its own.
Felt vs synthetic
Traditional underlayment is asphalt-saturated felt (15 lb and 30 lb weights): the tar paper of every old roofing story. It works, and it has real limits: it tears in wind, wrinkles when damp, and degrades faster with exposure.
Modern synthetic underlayment is a woven polymer sheet: lighter, far stronger, flatter-lying, longer-lived, and safer for crews to walk. It costs more per roll and has become the standard on quality installs for the same reason architectural shingles did: the per-year math wins. Synthetics are what we install.
Ice and water shield: the heavy artillery
Regular underlayment is water-resistant and nail-punctured by everything above it. Ice and water shield is the upgrade for the zones that need true waterproofing: a rubberized membrane that adheres directly to the deck and seals itself around the shanks of nails driven through it.
It earns its keep wherever water concentrates, lingers, or backs up: valleys, eaves (where ice dams form in our occasional cold snaps), and around penetrations and chimneys. Self-sealing around nails is the quietly brilliant part: the layer stays waterproof despite being fastened through thousands of times.
Why this layer is where cheap bids hide
Underlayment is invisible at the final walkthrough, which makes it the perfect place to quietly save money: lighter felt, membrane only where code minimums require, or 'reuse what's down there' on partial work. The roof looks identical on day one. The difference reports for duty during the first real storm, and annually thereafter.
It's why we keep saying compare scopes, not totals. The underlayment line items are short to read and long to matter.