A vent stack is a vertical pipe in a plumbing system's drain-waste-vent network that supplies outside air to the drainage pipes so trap seals aren't pulled dry by suction. It connects near the base of a drainage stack and runs up through the roof, distinct from a stack vent, which extends a soil or waste stack upward.
A plumbing drain system does not just carry waste downhill. It also needs a path for air to move the other way. That path is the vent stack, a dedicated vertical pipe whose only job is moving air, not water or waste. Without it, draining fixtures would create suction strong enough to pull the water seal out of every trap connected to the line.
The vent stack ties into the drainage system near the base of the drainage stack, the vertical pipe that actually carries waste down toward the building sewer. From there it runs upward, usually all the way through the roof, where it opens to outside air. As fixtures drain, that open path lets air flow in behind the falling water. Without it, the water would have to pull its own air supply from somewhere else in the system, such as back through a fixture's trap. Code requires a true vent stack on any drainage stack serving five or more branch intervals, roughly a five-story building or taller. Smaller residential systems usually vent through a simpler method instead, called a stack vent.
That distinction trips people up, because vent stack and stack vent sound almost the same and both are vertical vent pipes. A stack vent is just the upward extension of a soil or waste stack above the highest fixture connected to it. It is part of the drain pipe itself, simply continuing up and out through the roof. A vent stack, on the other hand, is a separate pipe built just for venting, tied in near the bottom of the system rather than continuing the drain pipe upward. Picture a two-story house with one bathroom stack: that stack vent alone usually meets code, while a ten-story apartment building needs a true vent stack to keep enough air moving through its much larger, harder-working drain system.
Either way, the goal stays the same. Keep enough air pressure on both sides of every trap so draining water never siphons the trap seal dry. A vent that is undersized, blocked, or ends in the wrong place can starve the system of air almost as badly as having no vent at all. That is why a gurgling drain or a sewer-gas smell often points back to a vent problem instead of the trap itself.