In order to predict the response time of smoke detectors in enclosure fires, a computational model is developed for calculating the local particulate concentration near the ceiling. The large scale smoke movement is approximated by integral equations for plume and ceiling-jet, which originates in the cold lower layer and penetrates into the accumulated smoke layer in the upper portion of enclosure. The effect of coagulation, which changes the particle size distribution, is included to enable predictions of ionization smoke detector response. This engineering model is designed to be used in combination with two-layer zone models for obtaining more detailed information of smoke concentration in the upper layer. Sample calculations have been made and comparisons with relevant experimental data showed a reasonable agreement both in the mass -jet.