When fixture drains connect at different levels sharing a common vent, which fixture is not permitted at the uppermost level?

When fixture drains connect at different levels and share a common vent, the configuration and plumbing codes dictate certain limitations to ensure proper drainage and venting. In this scenario, having a water closet (toilet) at the uppermost level is not permitted.

The primary reason for this restriction lies in the way a water closet functions. A toilet ejects waste water with considerable force, creating a significant amount of backpressure in the drainage system. Positioning a toilet above other fixtures can lead to complications. If the toilet is at the highest level, it may not be able to effectively drain without the risk of creating siphoning issues or affecting the drainage of the lower fixtures. The height difference can also interfere with the proper venting needed for all fixtures, potentially leading to odor issues or blockages.

In contrast, fixtures like sinks, showers, and bathtubs typically do not impose the same backpressure and are designed to drain more consistently without adversely affecting the plumbing of fixtures located below them. Therefore, in a system where multiple fixtures share a common vent, the toilet’s position is ideally lower to mitigate potential plumbing problems and ensure efficient operation.