Underlayment may refer to:
Underlay, a material placed underneath floor carpet, other flooring materials, or mattress bedding Underlayment, a water-resistant or waterproof layer used beneath many types of commercially available roofing material Bituminous waterproofing, systems designed to protect residential and commercial buildings SDM strength film, a kind of High Density Polyethylene (HDPE) Cross Laminated Strength Film Tar paper, a heavy-duty paper used in construction Underlayment in road construction such as nonwoven fabric Underlay, a material in sewing Underlay, a technique in some types of surgery such as myringoplasty Underlay, a material in printing