Security is recognized as one of the most important problems facing the wider use of hydrogen and the increased risk of accidental release into the infrastructure. Prismatic cavity design can be represented one of the best solutions for this problem. For this reason, dispersion and accumulation of hydrogen in a prismatic cavity with natural ventilation are computationally investigated by the commercial software FLUENT. In this paper, the roof apex angle A and the opening ventilation of a prismatic residential garage are examined to prove their effect on the hydrogen concentration gradients and stratification. The apex angle is varied from 180 degrees to 90 degrees and results have shown that the hydrogen mole fraction and the flow structure are strongly influenced by A, and that the lowermost concentration is obtained for A = 120 degrees. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.