Rock fall hazard assessment and hazard mapping are essential for the risk management of vulnerable areas. This paper analyses some issues concerning fragmental rock fall hazard mapping methodologies. Two Swiss approaches based on rock fall trajectory simulations results are presented. An application to a site in Switzerland emphasises the differences in the results, uncertainties related to hazard zoning procedures and the influence of some factors on the mapping process. In particular, the influence of a change in the temporal rock fall frequency, of the longer propagation along the slope of only a few computed blocks (defined in this sense as “extreme blocks”) and of the number of runs performed in trajectory modelling have been studied. Results are discussed with the purpose of achieving a more reliable and objective hazard analysis. The presented considerations are based on the Swiss Federal Guidelines, but many of them could be extended to other countries that evaluate rock fall hazard using an intensity-frequency diagram.