a process of simultaneously molding either a plurality of shingles or double walled shingles or other substantially planar exterior building products is provided , such that a blow moldable thermoplastic material is provided , of a composite parison that is then blow molded in a hollow mold , to produce novel shingles according to the process . ventilation and / or insulation can be provided to a hollow interior between spaced apart walls of a double walled shingle .