With few exceptions, all horizontal drainage piping shall have a cleanout at what location?

The correct answer is that all horizontal drainage piping shall have a cleanout at its upper terminal. This requirement is in place to ensure that proper access can be provided for maintenance and cleaning of the drainage system. The upper terminal of the horizontal drainage piping is typically where the system connects to vertical stacks or where the drainage system exits the building. This location is crucial because it allows for easy access to clear potential blockages and perform necessary maintenance without excessive labor or disruption.

While cleanouts can be beneficial at various other points in a drainage system for maintenance purposes, the requirement for having one at the upper terminal helps to ensure that any issues can be addressed efficiently. This is particularly important since blockages may occur in horizontal runs that lead to vertical systems, requiring access at points that are most likely to encounter such problems.

Additionally, positioning cleanouts only at the basement level, at every joint, or only at the end of the line may not provide adequate access to facilitate cleaning and maintenance effectively. The location at the upper terminal strikes a balance between accessibility and practicality in drainage system management.