Underlayment

Underlayment is the protective sheet — felt or, more commonly now, a synthetic membrane — rolled over the wood roof deck before the shingles go on. It's the roof's backup waterproofing: if wind-driven rain or an ice dam gets past the shingles, the underlayment keeps it from reaching the deck and the rooms below. In cold climates, code often requires a self-adhering ice-and-water membrane along the eaves. Underlayment is cheap on its own (a couple of dollars per square foot) but it's only installed during a re-roof, which is why a too-cheap bid that skimps on it costs far more later.