A self-adhering, multi-layer composite membrane for sealing a substrate and providing waterproof integrity to the substrate. The composite membrane includes: a surface layer of metal or of a polymeric material to interface the environment; a polyolefin film having a top surface and a bottom surface bonded to the surface layer by an adhesive at its top surface; a waterproof, asphalt-based layer adhered to the polyolefin film; a solid and continuous reinforcing mat without apertures therein having a top surface and a bottom surface, the top surface of which is adhered to the waterproof, asphalt-based layer; a second waterproof, asphalt-based layer adhered to the bottom surface of the reinforcing mat which is adapted to sealingly adhere to the substrate; and a polyolefin release film on the bottom surface of the waterproof asphalt-based adhesive layer to prevent adhesion between the asphalt-based adhesive layer and the surface layer when the composite is wound into a spiral roll.