A second-floor lavatory discharging through a vertical pipe without offsets from a kitchen sink is classified as what type of vent?

A second-floor lavatory discharging through a vertical pipe without offsets from a kitchen sink is classified as a wet vent because it allows both the lavatory and the kitchen sink to be ventilated through the same vertical pipe. In a wet venting system, one fixture serves as both a drain and a vent for another fixture. This occurs when the lavatory is located above the kitchen sink, allowing its discharge to flow down the vertical pipe that also serves to vent the sink.

Wet venting is often used in situations where fixtures are close to each other in elevation, which maximizes efficiency while minimizing the amount of piping needed. The vertical pipe essentially carries waste from the lavatory and also provides the necessary air circulation for the kitchen sink, preventing issues such as siphoning of traps.

In contrast, a dry vent would be solely for venting without any drainage capability, and a common vent would be more specific to fixtures sharing a drain line but not strictly in the manner presented here. A stack vent generally refers to a vertical vent that extends up through the roof independently. Hence, the classification as a wet vent accurately reflects the dual functionality of the system depicted in this scenario.