Flammable liquid spills or slow heavy vapor releases produce flammable layers which, if ignited can cause damage to the enclosure containing the release The question of pressure development in the ensuing explosion has been addressed in a previous paper This paper describes a model to predict the fuel distribution in the flammable layer as a function of the parameters that define the spill (or fuel release) scenario Since the model is intended for use in engineering predictions, it is designed to be easy to use and compatible with widely available computer resources Accordingly the flow field is treated as one dimensional taking only variations in the vertical direction into account This simplifying assumption is supplemented by special adjustments for effects not accurately addressed without the horizontal gradients This includes sub-models to deal with forced ventilation and cases where the vaporizing pool occupies only a fraction of the floor area of the enclosure The model has been validated by comparison with available data from vaporizing pools, and for diffusing heavy vapors Overall agreement with the experiment is satisfactory, and generally on the conservative side On the basis of these results, the formulation has been judged suitable for use in combination with the method to predict the pressure increase from combustion of the flammable layer These two elements are part of an integrated tool developed for use by FM Global Loss Prevention Consultants The paper discusses the implications of this modeling approach in terms of protection design.