A composite structural member, made of a polymer and wood fibre composite material, in the form of an extruded or injection moulded thermoplastic member in residential and commercial structures. The structural member can be used in a window or a door. The structural member has a hollow cross-section with at least one structural web (109) and at least one fastener web (110) formed within the member. The exterior of the extruded member is shaped and adapted for installation in a rough opening and to support window and door components. Such structural members can be assembled in a thermoplastic weld process. Welding is performed by heating and fusing the heated surfaces together to form a welded joint.