To limit the probability that an inadequate vertical height above the flood level rim of fixtures will lead to backflow into vent pipes in the event of blockages in drainage systems, which could lead to an obstruction of vent piping by soil or waste, which could lead to inadequate venting, which could lead to trap seal failure, which could lead to the entry of sewer gases into occupied space, which could lead to negative effects on indoor air quality, which could lead to harm to persons.