Discounting of escape stairways

Whether simultaneous or phased evacuation is used, where two or more escape stairways are provided it should be assumed that one of them may not be available due to fire or smoke. Therefore, each stairway in turn should be discounted in order to ensure that the capacity of the remaining stairways is adequate for the total number of people needing to use them. This discounting provision need not be applied where – (a) the escape stairways are approached on each storey (except the topmost storey), through a protected lobby; or (b) the escape stairways are protected by pressurization systems designed in accordance with BS EN 12101-6, however, discounting storey exits still needs to be considered (see paragraph 2.52).