How your plumbing system drains water

Granted, understanding how water flows from your sinks, tubs, and toilets is probably not high on your list of things to learn about. And you’re probably not losing any sleep over not knowing what happens to the solids that disappear with the water when you flush. However, this is all part of understanding how your house works. And when you know how it works, you make better decisions about it.

Once water flows through the plumbing fixtures (e.g., sinks, showers, toilets), it enters the plumbing drainage system, aka the DWV (drain, waste, vent) system. This system of pipes carries sewage and greywater (the used water that doesn’t contain sewage waste) from the home to an acceptable treatment location. 

As the wastewater flows through the drain pipes down through the house and out to the city sewer line or a private septic tank, there’s also a system of pipes that are connected to the plumbing that carries air up to the roof. This is the plumbing vent system.

When organic waste breaks down, toxic sewer gas is created. To keep that gas from seeping up through drains, every drain has a trap. In the plumbing world, a trap is a curved section of pipe that holds water. If the drain pipe holds water, sewer gas can’t get past the water. It “traps” the sewer gas in the pipe within the wall. 

The vent part of the system does two things: 1) It regulates air pressure within the pipes to prevent negative pressure from building up and siphoning out water from the plumbing traps, and 2) it allows sewer gases within the pipes to escape to the outside.

Drain Pipes
Starting at the plumbing fixture (sink, shower, toilet), the water enters a drain connected to a pipe. A drain could also be unattached to a fixture, such as a floor drain. The drain pipe can be made of metal or plastic. Metal could be copper, cast iron, galvanized steel, lead, or chrome-plated brass. 

Copper DWV pipes are rare but were used for a while, from the 1940s to the 1980s. Lead was popular among plumbers because it’s soft and easy to use. It was banned from use in plumbing systems in 1986. While lead drain pipes don’t affect your drinking water, if you have a leak in lead pipes, some jurisdictions won’t allow plumbers to fix lead pipes. Instead, they need to be replaced. 

Cast iron and galvanized steel are seen in older homes. They are both long-lasting, but each has issues. Cast iron can last about 80 to 100 years, but those homes are getting to an age where the pipes will start failing due to rust and corrosion. Galvanized steel has a coating of zinc to protect it from rust, but over time the zinc wears off on the inside of the pipe, and it rusts from the inside out. Same pattern of corrosion with cast iron - they rust from the inside out. Chrome-plated brass also wears away from the inside out, eventually leaving a thin shell of chrome that can crack very easily. 

Plastic pipes can be white (PVC), cream-colored (CPVC), or black (ABS). There were some problems with ABS piping made from 1984 to 1990 by certain manufacturers. Faulty resin in the pipe interacted with the cement plumbers use to glue joints together, resulting in weakened ABS pipes that cracked at the joints. If you have ABS plumbing from that era and it hasn’t leaked up to now, you’re probably fine. 

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P-Traps and S-Traps
Because sewer gases can be toxic (and very smelly), each drain must be “trapped.” A trap is a curved section of pipe that carries wastewater but retains enough water in the curved part to block sewer gases. The water acts as a trap, preventing noxious sewer gases from rising through the drain pipe and into your home.

The most common types of plumbing traps are P-traps and S-traps. However, S-traps were banned in 2006. Other than the shape (the P-trap is shaped like a “P” and the S-trap like an “S”), the main difference is that P-traps go through the wall to connect to the drain (and vent) pipe, and S-traps go through the floor. (Accept if it’s a P-trap with an air admittance valve - explained below).

The problem with S-traps is that if you have a lot of water draining through, the force of gravity on the water can cause the water in the curved section to siphon down into the drain pipe. This defeats the purpose of having water in the curved section to block sewer gases. If you have S-traps, you can solve this problem by pouring a cup or two of water down the drain after you’ve emptied a full sink or bathtub. Or, you might find that an air admittance valve and P-trap have been installed. This configuration doesn’t go through the wall but connects to the pipe going through the floor. 

Toilets have no visible trap, but one is actually there, built into the base of the toilet.

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Stacks and Horizontals
After the trap, the wastewater either continues down through vertical pipes or travels horizontally through a branch drain line and then connects with a vertical pipe. With horizontal pipes, even though they’re called horizontal drain lines, they actually need to be sloped 1/8 to 1/4 inch per foot of distance to allow for proper drainage.

A vertical section of drain pipe is called a “stack.” It transports both wastewater from the fixture and air to the fixture. It’s the same pipe (or they’re connected to the same pipe). Where a sink drain enters a stack, below that point it carries wastewater and is called a “waste stack.” Above that point, it carries air and is called a “vent stack.” A stack can vent multiple fixtures, but if one of those is a toilet, it’s referred to as a “soil stack.” It’s common for multiple vent stacks to connect together (usually in the attic) to limit the number of roof penetrations from plumbing vents.

All of the vertical and horizontal pipes tie together into one main drain line called the “house drain” that heads outside and ties into the city sewer line or to a septic tank. 

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Cleanouts

To help unclog drain pipes, Y-shaped cleanouts are provided. At a minimum, there will be a 4” diameter cleanout at the point where the house drain exits the building. This cleanout is helpful if tree roots invade the exterior drain line. You likely have additional cleanouts in other parts of the drain system. 

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Vents

If your DWV system has multiple vent stacks, you might see several plumbing vents on the roof. Alternatively, they could be tied together, and you’ll only see one vent pipe. Either way, it’s usually required that at least one vent stack vents to the outside. 

Keep in mind that the air exiting vent pipes is very humid, and in winter that water vapor can condense and freeze on the vent pipe as it exits the house. Over time, this can build up enough to block the vent. In areas with freezing winters, local building codes may require that the pipe be large enough in diameter to prevent ice buildup. Code may also require that the pipe extend past the roofline far enough to avoid being covered in snow.

Note: You might not have any plumbing vents through the roof. In some areas, it’s acceptable to run a plumbing vent to the outside of the house through an exterior wall instead of the roof. In that case, the vertical stack runs along the exterior of the house. There are usually some restrictions on the minimum height of the pipe and clearances to windows, etc.