Abstract: A system and method for building and assembling parts of or entire modular structures such as dwelling units, marine structures or watercraft from recycled PET material panels. Each assembly is made from panels of recycled PET material having a core and a rigid skin on both faces of the panel. The panels are joined together with splines of the same type of panel material. The splines may be glued and or fastened in place. The panels provide the necessary rigidity and insulative qualities for construction and watercraft manufacture and may be painted. The panels are impervious to environmental elements as well as to insects.