A composite structural member of the invention comprises a linear member having a first end and a second end attached to each end of the linear member as an end piece or end cap structure. Covering the composite member is a thermoplastic envelope preferably adherently bonded to the composite member. The end caps or end pieces are preferably thermoplastic materials typically thermoplastic composites comprising a thermoplastic resin and a fiber. Such a member is environmentally stable, resists moisture absorption, forms strong mitered joints and can be used in the assembly of fenestration products for commercial and residential real estate.