Driftwood racks are installed in rivers to hold back arriving logs at a hydraulically acceptable place, so that the downstream reach is protected from potential flooding. The bar spacing is a central rack parameter and was considered herein with physical modelling. Single driftwood logs (uncongested arrival) were supplied serially and their blocking or passage was counted and set in relation to the relative rack bar spacing and to the approach flow conditions. The data analysis, also considering approaches published in literature, provided an empirical estimation to predict the blocking probability.