A numerical implementation of a zone model which will transport fire, smoke and tozic gases in a multi-compartment structure is described. The areas covered are the equations which are solved, the numerical technique for the solution of these equations, species transport and the other relevant physical phenomena which govern fire growth and spread, and the transport of smoke. Also included in the model are the calculations necessary for a toxic hazard evaluation of a structure with a specific material loading. Forced ventilation is not yet included (version 15). A machine readable copy of the model (FAST) is available through NTIS. The tape includes the data file listed in the appendices of this report.