This paper describes the theory and some results of sample computer simulations on a model which is devised in order to predict the behavior of the hot gas generated by a fire source combustion when the fire breaks out in a small scale building.

This model computes the temperatures and depths of the hot gas layers in arbitrary rooms in a building in which a fire has broken out on the basis of the fundamental assumption that hot gas layers are formed and kept near the ceilings in any rooms into which the hot gas flows.

The author hopes that this model can be used to investigate the role that hot gas plays on fire spread from one room to another in small scale buildings such as Japanese dwelling houses. The application of the model to low buildings seems comparatively successful.