Underlayment

Underlayment is a layer installed beneath a finished surface to improve a flooring or roofing system. In flooring, it provides a smooth, level base for the visible floor, can reduce sound transmission, and may offer thermal insulation or a moisture barrier when used on concrete slabs. In roofing, underlayment sits between the roof deck and the exterior covering, adding a secondary level of moisture protection and temporary weather resistance during construction and after installation.

Flooring underlayment covers a range of materials and purposes. Foam or plastic sheets made from polyethylene or similar materials are common under laminate, engineered wood, and vinyl floors to smooth out small irregularities, dampen sound, and provide a moisture barrier on concrete slabs. Cork, felt, and rubber underlayments offer varying degrees of sound insulation and comfort. Some underlayments are self-adhesive or designed to accommodate radiant heating systems. In tile installations, a cementitious backer or decoupling membrane may function as part of the substrate rather than a traditional underlayment, improving crack resistance and adhesion.

Roofing underlayment exists to protect the structure when shingles or other final coverings are not yet installed or have failed. It includes asphalt-saturated felt and synthetic materials made from polypropylene or polyester, as well as self-adhering rubberized asphalt membranes. Roof underlayment should be resistant to moisture, compatible with the roofing system, and appropriate for the local climate, with attention to installation guidelines, overlap, and UV exposure.

Selection and installation depend on the flooring or roofing system, substrate moisture, and manufacturer recommendations. Proper installation ensures performance, durability, and the intended level of protection and comfort.