Underlayment
A layer of material installed between the roof decking and the finished roofing surface to provide a secondary moisture barrier.
Roofing underlayment is a water-resistant or waterproof sheet material installed directly on the roof decking before shingles, tile, or metal roofing is applied. Its primary role is to act as a secondary barrier against water infiltration if the finished roofing is breached. Standard felt underlayment (15 lb or 30 lb felt) has been used for decades; synthetic underlayments made from polypropylene or polyester are lighter, stronger, and more moisture-resistant.
In cold climates, an ice-and-water shield—a self-adhering rubberized membrane—is required by code along the eaves and in valleys to prevent damage from ice dams. Ice-and-water shield is also used around all penetrations (pipes, skylights) as an added safeguard.
In flooring, "underlayment" refers to a thin foam, cork, or cement board layer installed between the subfloor and the finish floor to provide cushion, sound dampening, or a smooth substrate. The two uses of the term are unrelated except in name.
Real-World Example
The roofing crew installed synthetic underlayment over the full roof deck, plus ice-and-water shield along the first 6 feet of eave to meet the local code requirement for a cold climate.