For the design of fire safety systems, evacuation analysis structural response and external façade performance, the heat release rate history is a primary factor regarding the development of building fires starting from the room of fire origin. This work examines the steady state heat release rate period in enclosures that occurs after the initial growth period ends because either fire has spread to all fuel available or ventilation- controlled conditions have been established. It extends previous important work by identifying how fuel type (e.g. stoichiometric ratio), fuel surface area, room geometry and opening affect the rate of burning. Using recent well-designed experiments in cubic like enclosures and in corridors together with a simple theory, correlations are developed for the rate of pyrolysis, of incoming air flow and of excess pyrolysate. In the course of the analysis, critical areas of research are suggested concerning combustion efficiency, heat fluxes, and effective fuel area involved in pyrolysis