At what diameter does the minimum size of any building drain serving a water closet need to be?

The minimum diameter of a building drain serving a water closet needs to be 3 inches to ensure proper function and compliance with plumbing codes. This size is established because a water closet, which typically requires a larger volume of waste to be removed effectively, generates significant flow and potential blockages.

A 3-inch diameter drain is designed to accommodate the discharge from a water closet, minimizing the risk of backups and ensuring efficient drainage. Additionally, this size provides sufficient capacity to handle the anticipated flow rates, taking into account the peak loading conditions that might occur in a residential or commercial setting.

Choosing a diameter smaller than 3 inches could lead to drainage issues, such as increased potential for clogs and inadequate flow rates. Therefore, adhering to this minimum size helps in maintaining a reliable plumbing system that meets safety and performance standards.