How must the portion of the stack above an offset be sized according to IPC regulations?

The portion of the stack above an offset must be sized based on the total number of drainage fixture units above the offset according to the International Plumbing Code (IPC) regulations. This requirement ensures that the stack can adequately handle the expected drainage load from the fixtures connected above the offset. Proper sizing is critical for preventing overflow and allowing for effective drainage by accommodating the volume of wastewater that will flow through the stack.

Sizing the stack in this manner helps ensure compliance with local plumbing codes, reducing the risk of plumbing issues that could arise from inadequate capacity, such as slow drainage or sewage backflow. This principle is fundamental in plumbing design, as it promotes proper hydraulic performance within the drainage system, taking into account the specific usage of the plumbing fixtures served by the stack.