Fire risk for building occupants is determined by the time required for safe egress and the time at which untenable conditions develop in building rooms or on the egress routes. In the present study, the CFAST zone model is applied to the simulation of fire development and smoke propagation in multiroom buildings, with the aim of studying the sensitivity of predicted tenability times and quantification of the uncertainties due to variability of the scenario parameters. The building geometries considered are (i) a single room with a door and a window, (ii) three rooms connected by a long corridor, and (iii) two-level configuration with two rooms on the lower floor connected by a vertical vent to a room on the upper floor. Sensitivity indices of the tenability times due to high temperature and smoke obscuration are analyzed. Distribution functions of the tenability times are presented and compared for different building rooms.