A wall and/or ceiling polymer-based composite structural sheathing board has a polymer material or predominantly polymer material core layer with a density between 1.6 lbs/ft3 and 25 lbs/ft3. A facer overlays at least one of the major surfaces of the core layer. The facer is generally coextensive with and bonded to the overlaid major surface of the core layer and enhances the integrity and fastener pull through strength of the polymer-based composite structural sheathing board as well as other desired physical and performance characteristics of the polymer based composite structural sheathing board. A wall and/or ceiling system of a building structure includes a plurality of the polymer-based composite structural sheathing boards overlaying and secured to a structural wall and/or ceiling frame and forming a wall and/or ceiling sheathing layer over the structural frame.