Underlay or underlayment[1] generally refers to a thin layer of cushioning made of materials such as sponge rubber, foam, felt, or crumb rubber; this material is laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against sound, moisture, and heat.[2] In general, it is a layer which is underneath another layer,[2] 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;[1] this roofing membrane is often made of rubber and is used to seal the roof and prevent leakage.


Popular carpet underlays are

  • foam rubber
  • crumb rubber
  • felt

Carpet underlays are typically 6-10mm. They primarily provide foot comfort, but they also reduce carpet wear and provide sound and thermal insulation.

Wood floorEdit

Underlay for timber floors is typically 3mm closed cell plastic foam. This primarily provides sound insulation and a vapour barrier.


  1. 1.0 1.1 "Underlayment." The Oxford English Dictionary. 2nd ed. 1989.
  2. 2.0 2.1 "Underlay." The Oxford English Dictionary. 2nd ed. 1989.
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