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The study is devoted to the generalization of the experimental results obtained by means of investigation of the quantitative characteristics of the modeled flammable hydrogen-air cloud which can occur in case of emergency leaks of hydrogen in underground or multi-story parking lots of hydrogen transport. Data are presented on 28 runs of light helium gas (hydrogen substitute) during its vertical propagation and interaction with an extended ceiling in case of two main types of outflow - jet and plume. The analysis of the experimental data by varying the parameters of helium release revealed different types of asymptotic decline of helium concentration in the under-ceiling layer along the radius of spreading, as well as different behavior of the under-ceiling layer thickness. The consequence of this difference was that, in the case of plume outflow, much larger volumes of the simulated fire-hazardous mixture were observed in the under-ceiling layer than in case of jet outflows. Some qualitative analytical explanation of the observed phenomena was proposed, that could be used to establish quantitative theoretical dependencies useful for development of the engineering guidelines on safety of the semi-confined and confined parking areas for hydrogen-fueled vehicles.