Which part of the waste stack extends above the highest horizontal drain connected to the stack?

• A. Cleanout
• B. Vent Stack
• C. Overflow
• D. Stack Vent

Answer: (D)

The key idea is venting the drainage system so traps don’t siphon and drains won’t push gases back into the building. The part of the waste stack that continues above the highest horizontal drain connected to that stack is the stack vent. It rises above all connected fixtures to provide an opening to the atmosphere, allowing air to enter as water drains and giving sewer gases a path out. That positioning is essential for preventing negative pressure from pulling water out of traps and for keeping the whole system draining smoothly.

A cleanout is just an access point for clearing blockages, not for venting. An overflow is a protective feature on fixtures to prevent flooding, not part of the venting path. A vent stack refers to the vertical venting system, but the portion that extends above the highest horizontal drain is specifically the stack vent.