The lattice Boltzmann method (LBM) is becoming an effective numerical technique of computational fluid dynamics (CFD). In this study, with some new thermal LBM schemes being proposed, the LBM is used to simulate the gravity current prior to backdraft (a particular and hazardous phenomenon in compartment fire) within laminar restrictions. The dimensionless time for gravity current traveling from the opening to the rear wall of a bench-scale compartment is calculated under different opening geometries, respectively, including: full end opening, upside-slot end opening, middle-slot end opening, downside-slot end opening, and slot ceiling opening. The application is very successful and the results show that the dimensionless time under the slot ceiling opening is the longest. Among the slot end openings, similar dimensionless time has been obtained for the upside-slot and middle-slot end openings, which is shorter than the downside-slot end opening. For the full end opening, the shortest dimensionless time is obtained. Finally, some valuable advices are given for fire protection engineering. Copyright © 2010 John Wiley & Sons, Ltd.