What should be the size of a vent pipe relative to the diameter of the drain it serves?

The vent pipe should be at least equal to the size of the drain it serves to ensure proper ventilation of the plumbing system. Venting is critical for balancing pressure in the drainage system, allowing wastewater to flow smoothly and preventing the formation of vacuum conditions that could hinder drainage. If the vent pipe is undersized, it may not effectively allow air to enter, which can lead to slow drainage and potential backups.

By having a vent pipe that is at least equal to the diameter of the drain, adequate airflow is maintained, which supports the drainage system's efficiency and performance. This practice aligns with plumbing codes and standards which are designed to ensure safe and effective plumbing installations.