If two lavatories are installed back to back in a wall, what is true about their waste connections?

When two lavatories are installed back to back in a wall, it is acceptable for each fixture to be connected to a separate waste line. This configuration allows for efficient plumbing design, maintaining proper drainage and venting for each sink while providing flexibility in maintenance or future modifications.

Connecting each lavatory to its own separate waste line ensures that the flow and drainage are independent, which can be advantageous in case one sink experiences a clog or blockage. Additionally, separate connections allow for more precise venting and are often more compliant with plumbing codes, which mandate that fixtures should have adequate drainage and venting to prevent siphoning and ensure proper function.

Although sharing a single waste line (which is an alternative configuration) can be possible in some contexts, having separate connections is generally more favorable for operational integrity. Hence, the conclusion that each lavatory may be connected to a separate waste line aligns with best practices in plumbing installation while ensuring compliance with regulations.