Identifying the Parts of Your Home's Drain (DWV) System

Close-Up Of Sink
Bernard Van Berg / EyeEm / Getty Images

When it works correctly, we give no thought at all to various components of a home's drainpipe system. But when it doesn't work, you must be able to identify and locate the parts very quickly. Now is the time to learn about your drain system—not when a drain pipe has failed and is spilling dirty water all over your bathroom floor. 

While we normally think of this system simply as "the drain pipes," to be technically correct, the system is known as the DWV system. The acronym stands for Drain-Waste-Vent, and it indicates that this pipe network is not only designed to drain wastewater and solid wastes to the municipal sewer or septic field but also serves as a vent system that allows fresh air into the drain system.

To learn about the drain system, it can be helpful to trace a drain line from a single plumbing fixture, such as a bathtub or sink, all the way out to the municipal sewer main. 

  • 01 of 10

    Fixture Drains

    Picture of tub drain removed

    Lee Wallender

    The upper, visible section of a tub, shower, or sink drain is what everyone is familiar with. Each drain pathway begins with an opening in the fixture, often fitted with a plug or stopper, where wastewater begins its journey from the fixture onward to the sewer lines or septic field.

    While this is the most obvious component, it is relatively rare for drain problems to originate here. Except when gaskets or washers may cause a tub or sink bowl to leak, the most common problems—drain clogs—almost always occur downstream from the fixture drain openings. The one exception: when hair clogs a pop-up stopper in a bathroom sink or bathtub. 

    From here, wastewater drops down below the fixture to the next component—the drain trap.

  • 02 of 10


    Immediately beneath a sink, bathtub, or other plumbing fixture, the fixture drain opening leads to a curved segment of pipe known as the P-trap, which is normally a 1 1/4 to 2-inch-diameter segment of pipe with a sharp curved bend in it, shaped like the letter "P." 

    The purpose of this drain trap is to hold standing water, which seals the drain system and prevents sewer gases from rising from the sewer system into your home. If you've ever returned home after a long vacation and noticed a faint sewer gas odor in the air, it's probably because the standing water in the drain traps has evaporated, allowing the sewer smell into your home. 

  • 03 of 10

    Toilet Trap

    Although it is not immediately evident, every toilet has built into it a curved drain trap of its own, which you may be able to see if you view the toilet bowl unit from the side. This built-in trap serves the same function as a sink drain trap: to trap water and keep sewer gases from rising into the home. 

  • 04 of 10

    Clothes Washer Stand Pipe

    Washer Drainage Line

    Lee Wallender

    Another specialty form of drain trap is the one that serves your clothes washer. The washing machine drain tube empties into an exposed standpipe that leads down to a curved drain trap, which in turn leads to a branch drain and onward to the main drain. Most of these parts may be hidden behind finished walls, but the standpipe itself is often exposed. 

    Depending on the age of your plumbing installation, the standpipe system can be made of galvanized iron, brass, or PVC or ABS plastic. 

    Continue to 5 of 10 below.
  • 05 of 10

    Branch Drain Lines

    Branch Drainage Line
    Branch Drainage Line

    Lee Wallender

    Branch drain lines run horizontally (though with a slight downward slope, called pitch, to connect each of the fixture drain traps to soil stacks, which in turn will lead to main drain lines. Branch drains are often completely hidden by the finished wall, ceiling, and floor surfaces. They can be made from a variety of materials, and are usually 1 1/2- to 2-inch diameter pipes.

  • 06 of 10

    Soil Stack

    As branch drains reach the end of their horizontal runs, they empty into soil stacks, or main drain stacks, which are large-diameter vertical pipes. Wastewater, along with the solid wastes that they carry, will now drop down into the main drain lines leading out to the city sewer system or septic field. 

    Soil stacks are larger pipes, typically 3 to 4 inches in diameter. 

  • 07 of 10

    Soil Stack Vent

    The upper part of the vertical soil stack provides the venting component of the DWV system. If you follow it upward, the vent stack penetrates through the roof of your home, where it is open to the outside air. The vent allows the entire drain system to maintain equal air pressure. This is essential to prevent the suction power of water moving through the pipes from pulling water out of the individual drain traps.

    If you have ever heard a drain gurgling as you empty water, you are hearing the sound of a slight air-pressure vacuum attempting to pull water out of the drain traps. If the vent system is working correctly, this suction never reaches a point where water is sucked out of the traps. 

    The vent pipes serve two functions:

    1. It carries noxious fumes away.
    2. It provides a pressure release so that discharge waste and soil can move downward easily, without pulling water out of the drain traps.
  • 08 of 10

    Sewer Line Clean-Out

    The sewer clean-out or main house trap is only there for emergencies and regular cleaning. It is usually found in a cap or hub mounted to a 3- to 4-inch diameter pipe that rises out of a slab floor in a basement or utility area. Sometimes it is mounted flush into the floor. In some climates, the clean-out is mounted in an in-ground fitting located just outside the foundation of the home. 

    This fitting is used when a major clog is blocking the main sewer line. With the cap removed, a motorized auger can be used to remove a clog in the main drain.

    Continue to 9 of 10 below.
  • 09 of 10

    Main Drain Line

    Types of sewer pipe

    Lee Wallender

    All wastewater from your house is carried to the municipal sewer line by one main drain pipe that typically runs horizontally, but with a slight downward slope, under the lowest floor in your home out to the municipal sewer main or out to the septic field. In cases where the street sewer or septic field is not low enough, the main drain pipe could be hanging along the wall in your lowest floor before it leaves the structure.

    Usually 4 inches in diameter, this pipe might be either ABS or PVC plastic, clay, or cast iron. The mainline is rarely visible since it usually runs under the home's basement or foundation slab. You will probably see this drain line only during major repairs or additions to the system.

  • 10 of 10

    Municipal Sewer Main

    Municipal Sewer Line

    Lee Wallender

    The end point of your home's drainage system is the municipal sewer main. Your home's main drain line runs perpendicular into the sewer main, angled downward to promote the flow of waste. You have no control over the municipal line since it is owned by a city, county, or wastewater district.

Article Sources
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  1. Sewer Division. City of St. Clair Shores, Michigan.