Underlayment is a water-resistant layer installed between the roof deck and the visible roofing material, acting as a secondary barrier against water intrusion.
Even a well-installed shingle, tile, or shake roof isn't fully waterproof on its own — wind-driven rain, ice dam backup, and minor material failures can all let some water past the surface layer. Underlayment catches that water before it reaches the wood decking underneath.
Felt underlayment (traditional asphalt-saturated paper) is the lowest-cost option. Synthetic underlayment is more tear-resistant, lighter, and increasingly the standard choice. Self-adhered (peel-and-stick) underlayment, often called ice-and-water shield, is required in many cold climates at the eaves and valleys for extra ice dam protection.
Yes — underlayment is installed fresh with every full reroof, since it's beneath the shingles/tile and can't be accessed or inspected without removing the surface material.
Synthetic underlayment can last 20-30+ years if never exposed to UV (which happens if the surface material is removed and not promptly replaced); felt degrades faster, typically matched to the shingle's own lifespan.
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