What is a Soil Stack?

M. McGee

A soil stack is the part of a plumbing system that connects interior plumbing to the sewer and vents gases out of a building. These pipes may be found in any building type, but they are in nearly every residential home or small building. When a system uses a soil stack, the pipe runs vertically from below the building to a vent on the roof. These vertical pipes are characterized by an occasional whooshing sound as waste water flows downwards from an upper drain.

The bottom of a soil stack furnishes an exit to either a sewer or a septic tank.
The bottom of a soil stack furnishes an exit to either a sewer or a septic tank.

When waste water leaves a toilet, sink, or bathtub, it flows through a waste water pipe to a centrally-located soil stack. Most residential buildings use a gravity-based system, and the waste pipes are always slanted downwards to assist in waste removal. Once the water hits the vertical soil stack, the solids and liquids travel down and the gases travel up.

At the bottom of a soil stack, there is an exit to a waste-removal system. This may be an exit to a sewer, if the building is in an urban area, or to a septic tank when the building is rural. This pipe is also slanted downwards, like the smaller in-house waste pipes.

The upper end of the soil stack is usually just an open pipe. This vents gasses out to the atmosphere, where they dissipate almost immediately. Some of these vents have coverings that prevent birds or insects from having full access to the pipe, but these are often unnecessary, as the smell alone deters most creatures.

Since the top of the soil stack is open to the air, it is a neutral pressure system. The air pressure inside the pipes is the same as the air pressure outside. This allows the solids and gasses to move in separate directions without creating a suction or siphon effect.

When the air vent is blocked, the pipe’s pressure begins to increase. Eventually, this will result in sewer gases venting through the easiest opening into the house. Often this will start with the highest drains in the system, which are the ones closest to the blocked vent. This will alleviate the pipe’s pressure and allow solids to leave the system, but it will fill the home with harmful and foul-smelling gases.

Soil stacks are common in nearly every home in North America and many other developed parts of the world. This waste-removal method is also common in nonresidential buildings with simple plumping systems, like small stores, restaurants and so on. More complex systems, like hotels, office buildings or large apartment buildings, may use a different waste-removal process.

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