fleece

In construction, fleece is a nonwoven mat made from continuous or staple fibers, typically polyester or polypropylene, bonded together through heat, needle-punching, or chemical adhesive. It serves as a backing layer for roofing membranes, waterproofing sheets, and vapor barriers, protecting them from puncture and providing cushioning against substrate irregularities.

The primary function is damage prevention. Without fleece backing, a sharp aggregate, wood splinter, or concrete protrusion can tear or puncture the membrane during installation or under foot traffic. Typical fleece weights range from 100 to 500 grams per square meter (gsm), with heavier grades offering more impact protection but less flexibility. Single-layer membranes often come factory-bonded with fleece on one or both sides.

Common applications and variants

Fleece appears most frequently in self-adhesive EPDM and PVC membranes for flat roofing, and as a separator in multi-ply systems. It is also used in thermoplastic polyolefin (TPO) sheet goods and under liquid-applied waterproofing membranes to prevent direct contact with substrate porosity. In some cases, contractors use loose or unfaced fleece as a temporary protective layer during construction before the actual membrane is installed.

The main deterioration risk is moisture entrapment. If water reaches the bond line between fleece and substrate without escape routes, the adhesive can fail or the backing can degrade over years of exposure to freeze-thaw cycles. Specification of fleece thickness must balance protection against water vapor transmission and the substrate's ability to dry.

The term itself reflects the material's appearance and origin: the texture resembles natural fleece, and the technology emerged from textile manufacturing processes. In British and European practice, the equivalent material is sometimes called a separator layer or non-woven fabric, though fleece has become the standard term in North American construction standards and product data sheets.