Abstract: A fire resistant floor and ceiling assembly comprising in combination; a plywood layer; a gypsum floor underlayment extending adjacent below said plywood layer; a series of steel joists located below said underlayment and supporting the plywood and underlayment; a space-apart multiplicity of self-drilling self-tapping screws extending downwardly through the plywood and underlayment, and engaging said steel joists thereby attaching the plywood and underlayment to the steel joists, said screws having upper threads and lower threads with an interrupting non-threaded mid-portion therebetween, the non-threaded mid-portion extending a distance substantially the same as the thickness of said plywood layer; and, at least one layer of third generation fire resistant gypsum board connected to, and supported below, said steel joists by connecting means; whereby said assembly contains no combustibles in the plenum between said underlayment and fire resistant gypsum board, and whereby said assembly attains at least a one