An integrally molded plastic structural unit includes spaced support members joined by a web section disposed therebetween, each of the support members including spaced face sections and an intermediate section therebetween. Each face section includes a stressed hard outer skin. The intermediate section includes a peripheral frame section of high density. The web section includes high density connector members extending between adjoining face sections of the spaced support members. Each of the support members includes mateable sections along at least two edges thereof, with each of the mateable sections including at least five alternating raised and recessed areas forming an asymmetrical pattern along the length of each mateable section. The pattern is the same or a reverse pattern thereof along each mateable section of the same unit and of adjacent units with which it will be assembled. The mateable sections extend beyond the face members or are recessed below the face members on the edges of the unit a substantially equal distance. Also, a method of molding the structural unit.