The present paper discusses the mechanism of the wind-induced scattering of permeable unit flooring decks loosely laid on the rooftops and balconies of high-rise buildings. Firstly, the scattering mechanism of decks was investigated, based on a blowing test using a blower and actual decks. Subsequently, a simulation model was constructed for estimating the internal pressures under the decks (pressures acting on the bottom surface of decks) obtained from a wind tunnel experiment – this is based upon the unsteady Bernoulli equation and the time history of external pressures on the rooftop and balconies. Combining the simulated internal pressures and the experimentally obtained external pressures, the time history of net wind pressures acting on the decks were computed. Finally, the threshold wind speed of scattering (scattering wind speed) was obtained by applying the scattering mechanism to the simulated time history of the net wind pressures.