Abstract: A flexible membrane having a valuable combination of desirable properties is composed of a generally heavy, dense supporting and reinforcing reticulated base fabric constituted of thick, generally loose bundles of multiple continuous filaments arranged in a mechanically interengated reticular array having an overall weight within the range of about 3-12 oz/yd.sup.2 the continuous filaments being of a synthetic polymer having good dimensional stability and high resistance to heat and light; and a solidified base coating composition completely impregnating the interstices of the base fabric and as forming a continuous coating along the opposite sides thereof, the composition having as essential ingredients a PVC resin and a plasticizer therefor in the amount of 35-75% of the resin of a trimellitate ester which exhibits high resistance to separation from the resin and imparts to the coating high flexibility at temperatures at least as low as about -35.degree. F.