What Is a Soil Stack?

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.

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.

To understand how soil and waste stacks work, let's take a look at greywater for a moment. Greywater comes from bathtubs, showers and sinks plus washing machines and other appliances — but not toilets. It has not yet come in contact with toilet water or feces, hence why it's called "grey." On the other hand, water from toilets takes a slightly different route. Each toilet has a pipe that eventually leads to the soil stack, not the waste stack.

Assuming you have multiple stacks in a drain-waste-vent system, greywater typically goes right to the waste stack. Here's where we can see the difference between the two stacks. They may run parallel to each other, but they don't touch. In this two-stack configuration, they empty in the main drain and then flow to the sewer.

There's one other key difference between soil stacks and waste stacks: size. Most soil stacks use a wider pipe than waste stacks. With a wider pipe, solid waste can more easily flow down into the building drain toward the sewer. Municipal plumbing codes often specify minimum pipe diameters both for soil and waste stacks.

Some DWV systems use wet vent stacks. In this case, each wet vent stack does double-duty as a waste pipe and a vent. These wet vents can allow water to drain and let gasses out at the same time. Most systems using wet vents have just a single drain stack that handles all the outgoing water.

Because each wet vent connects right to the main stack, all the water from those vents flows into the main stack before heading to the sewer. Greywater doesn't stay "grey" for very long before combining with soiled water from toilets. You'll often find multiple fixtures on each floor connected to a single wet vent. Municipal plumbing codes specify many design aspects of a wet vent, including pipe diameter and how they connect to horizontal drain lines.

With a wide range of home and room designs, creative plumbing solutions may be required. Loop vents are one excellent example. They're typically installed in places where it's impossible to run a vent directly into a wall. Kitchen island sinks commonly use loop vents, for example.

Like wet, soil stack and waste stack vents, loop vents also let gasses escape. The principle's pretty much the same: Gasses flow up while water flows down. From the sink, the trap pipe empties into a vertical drain pipe with a series of elbows connected up at its top. The elbows create a curve or loop, with gasses vent up and through the loop to a vertical vent pipe that branches to both a vent line in a nearby wall and to a drain line below.

In a loop setup, the vertical drain pipe allows water to flow down into the horizontal drain line beneath the floor. Both the vertical drain and vent pipes are linked to the drain line via 45-degree elbows connected to straight pipes. The straight pipes then reach the drain line through 45-degree wye connectors. The horizontal drain line empties either into a waste, soil or main drain stack. State and local plumbing laws govern where and how loop vents should be placed.

Drain-waste-vent systems can be configured in many different ways. We've already mentioned two-stack systems that use separate waste and soil stacks to drain water and vent gasses out. There are single-stack setups that use an additional venting pipe to let gasses out and up. Many modern homes use a single-stack system, while older houses usually rely on double-stack plumbing setups. However, this isn't a hard-and-fast rule.

As mentioned earlier, soil stacks send waste out of the building and vent gasses up. A single main track performs the same purpose except without a separate stack for solid wastes from toilets. Regardless of your plumbing system's design style, you will need at least one stack to vent gasses and drain water out of the building.

Both single- and double-stack systems must eventually lead to the horizontal main drain pipe beneath your building. Your DWV system terminates with the main drain line. This line runs horizontally but slopes somewhat downward until it connects with the municipal sewer main. Alternately, the main drain line may lead to a septic tank.