Why should drainage stacks not be designed to be more than one-third full?

The rationale for not designing drainage stacks to be more than one-third full centers around the prevention of siphonage blowout. When a drainage stack is filled beyond this limit, it can create a scenario where the hydraulic conditions may lead to a siphon forming. This siphon can remove water from trap seals in plumbing fixtures, which are essential for preventing sewer gases from entering the living spaces. Ideally, maintaining the flow within one-third of the stack's capacity allows for sufficient air space to mitigate the possibility of siphonage, ensuring that fixtures remain sealed and free from unpleasant odors and hazards. Such design considerations are critical in plumbing systems to maintain their functionality and safety.