Underlay or underlayment generally refers to a thin layer of cushioning made of materials such as sponge rubberfoam, felt, crumb rubber, or recycled plastic; this material is laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against soundmoisture, and heat. In general, it is a layer which is underneath another layer, so underlay is thus also used to describe many different surface-covering products.

In vinyl flooring or "linoleum", the underlay is the thin layer of plywood that is fastened over the structural subfloor to create a uniform, smooth platform for the sheet vinyl. For laminated wood flooring, the underlay provides a “vapor barrier” to prevent moisture from coming through the floor of the home and then migrating into the flooring; the underlayment may also have noise-dampening properties.

A self-leveling underlay is a cement-like product that can be pumped in liquid form onto the floor in order to create a level floor.

Underlay is also the term for the material under roofing tiles; this roofing membrane is often made of rubber and is used to seal the roof and prevent leakage. Underlayment used with roofing shingles provides a second layer of water proofing to prevent leaks and is called tar paperroofing felt, or since the 1990s synthetic underlayment.

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