A plumbing drainage venting system is the component of a drainage system that maintains neutral pressure in the pipework, allowing flow of water down drains and through waste pipes. Venting also removes foul or combustive gasses from the system, usually through vents located above a building's roof.

The abbrieviation "DWV" (drain-waste-vent) or "SVP" (soil & vent pipe) refers to the piping of a building's complete drainage system.

Venting is important to prevent pressure differentials (trapped pressure or suction) in the system, which could compromise water seals in the piping.

A sewer pipe is normally at neutral air pressure (equal to the surrounding atmospheric pressure). That is, the pipe is not pressurized or producing suction. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or push back on the waste stream. As the column of water passes, air must flow in behind the waste stream or negative pressure (suction) results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.

Excessive negative air pressure can siphon water from trap seals at sanitary appliances (such as the p-trap under a sink). Generally, a toilet outlet has the smallest trap seal, making it most vulnerable to being emptied by induced siphonage. An empty trap can allow noxious sewer gasses to enter a home.

On the other hand, if the air pressure within the drain becomes suddenly higher than ambient, this positive transitent could cause wastewater to be pushed into the appliance, breaking the trap seal, with dire hygiene and health consequences.

Tall buildings are particularly susceptible to this problem.

To prevent this, sewer pipes will usually have a vent to atmosphere, air admittance valves, or HepvO valves. Each device has advantages and disadvantages; AAV's only work on negative pressure, and HepvO valves are relatively new. In any case, they all function by allowing ambient air to enter the system, keeping the system near ambient pressure.