Disclosed is a flooring system consisting of a slab (or base) floor, a floor covering, and a composite underlayment extending between the slab floor and floor covering. The composite underlayment may include, for example, one or more set gypsum layers with embedded fibrous mats. An exterior plastic coating is mechanically adhered to an underlying set gypsum layer. The plastic coating chemically bonds and cross-links with polymer additives within the set gypsum core. The result is an underlayment that is a fully integrated polymer matrix with greatly improved durability and surface strength with only minimal increases in cost or weight.