Abstract: A building assembly for efficiently and economically constructing walls and roofs for a building assembly that utilizes a prefabricated building panel. The prefabricated panels utilize miter-cuts when joined, form cores, interior and exterior walls, and roofs, when placed together. The walls are pre-formed, offsite according to the required size dimensions and then transported to the job site. The panels are made essentially of a expanded polystyrene panel having cores and channels. Each building panel also includes vertically disposed parallel voids that are approximately 4 in. in diameter, spaced approximately 2 to 4 ft. apart, and receive poured concrete and steel rebar to provide structural rigidity to the entire building assembly. The poured concrete includes rebars that protrude through the entire channel and coring system and are embedded in the slab, and which may be extended above the wall height to bend and overlap the roof for added structural support.