What is the common size for a building drain's cleanout?

The common size for a building drain's cleanout being four inches is based on standard plumbing practices and the diameter of the piping systems typically used in residential and commercial buildings. Cleanouts are essential components in a drainage system, allowing for easy access to unclog or maintain the building drain.

A four-inch cleanout is considered suitable because it provides a large enough opening to accommodate standard cleaning tools and equipment, facilitating efficient removal of blockages. This size aligns with the typical diameter of building drainage systems, which are usually designed to handle larger volumes of waste and prevent clogs.

While smaller cleanout sizes such as two and three inches are present in some plumbing systems, they are often used in smaller or less complex applications, and may not provide sufficient access for necessary maintenance tasks. The six-inch size, while functional, is less common and generally used for specific applications or larger plumbing systems. Thus, four inches is the optimal choice for building drains to ensure accessibility and functionality in maintenance practices.