Which of these fixtures requires an overflow?

A bathtub requires an overflow because it is designed to hold a significant amount of water. The overflow is a critical safety feature that prevents water from spilling over the edge of the tub, which could cause water damage to the surrounding area and create a slipping hazard. When the water level rises too high, the overflow directs excess water to a drain, keeping the water level within a safe range.

In contrast, while lavatories, shampoo sinks, and toilets have specific drainage designs, they often do not have the same level of water containment as a bathtub. Lavatories may also have an overflow feature, but it is not necessarily required as it is for bathtubs, which are more likely to be filled to the brim. Toilets have a different purpose and do not function with standing water in a way that necessitates an overflow, and shampoo sinks, while used in salons, are typically designed to drain quickly and do not need an overflow system in the same manner as bathtubs. This understanding highlights the special requirement for bathtubs to ensure both function and safety in plumbing code practices.