House Drain System: Parts and Diagram

Diagram showing the parts of a home's drain system

The Spruce / Candra Huff

When it works correctly, we give little thought to the various components of a house drain system. But when the system clogs, you must be able to identify and locate the parts 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. 

To learn about the house drain system, follow this guide and diagram from the start (fixture drains) to the end of the line (municipal sewer main).

  • 01 of 10

    Fixture Drain

    A fixture drain is the upper, visible section of a tub, shower, or sink drain.

    The fixture drain is the start of the drainage pathway or the DWV. The path begins with an opening in the fixture, often fitted with a plug or stopper, and travels 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 is when hair clogs a pop-up stopper in a bathroom sink or bathtub. 

    After the fixture drain, the wastewater drops below the fixture to the next component—the drain trap.

    Drain-Waste-Vent (DWV)

    Drain-Waste-Vent or DWV indicates that the 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.

  • 02 of 10

    Drain Trap or P-Trap

    The drain trap or P-trap is the curved segment of pipe directly below the fixture drain.

    The drain trap is below the sink, bathtub, or other plumbing fixture. The drain opening leads to the drain trap or P-trap. This pipe 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." 

    Drain traps are meant 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. Run the water at every available source and flush every toilet to fill up the drain traps again.

  • 03 of 10

    Toilet Trap

    A toilet trap is a curved drain that works much like a drain or P-trap on a sink or bathtub to prevent sewer gasses from entering the bathroom.

    The toilet trap is part of the toilet. 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 Standpipe

    A clothes washer standpipe is an exposed or hidden vertical pipe that assists with draining water from the clothes washer on its washing cycles.

    The flexible, plastic washing machine drain tube empties into a washer standpipe that leads down to a curved drain trap. The drain trap, 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, PVC, or ABS plastic. 

    Continue to 5 of 10 below.
  • 05 of 10

    Branch Drain Line

    A branch drain line is a nearly horizontal pipe (it is slightly pitched to move the water) that leads from the traps to the main drain line.

    Branch drain lines connect each of the fixture drain traps to soil stacks, which in turn lead to main drain lines. Branch drains are often completely hidden by the finished wall, ceiling, and floor surfaces. Sometimes, branch drain lines are visible in unfinished basement ceilings.

    Branch drain lines can be made from a variety of materials, and are usually 1 1/2- to 2-inch diameter pipes.

  • 06 of 10

    Soil Pipe

    A soil pipe is a vertical pipe within a plumbing system that moves soil, or sewage, down to the sewer line.

    As branch drains reach the end of their horizontal runs, they empty into soil pipes or soil vent pipes (SVPs), intended to move sewage to the larger sewer lines.

  • 07 of 10

    Soil Stack Vent

    A soil stack vent is a long vertical pipe that ends just above the roofline to remove foul vapors and gasses from a plumbing system.

    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.

    The vent pipes carry noxious fumes away and provide a pressure release so that discharge waste and soil can move downward easily, without pulling water out of the 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. 

  • 08 of 10

    Sewer Line Clean-Out

    The sewer line clean-out is an opening in the sewer line for cleaning out the sewer.

    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.


    It's important to keep the sewer clean-out operable in case you need it in the future. Once a year, open the sewer clean-out and then close it up again. This is especially important to do older metal or cast-iron clean-outs, since they can rust.

    Continue to 9 of 10 below.
  • 09 of 10

    Main Drain Line

    The main drain line is the large buried pipe that moves all of the house's sewage to the municipal sewer line.

    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 on 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

    The municipal sewer main is the end of your house's drainage system and the beginning of the municipal sewer system.

    The endpoint of your home's drainage system is the municipal sewer main. Your home's main drain line runs perpendicular to 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.

    Video sewer inspections are usually capable of running all the way to the beginning of the municipal sewer main.

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  1. Definitions of Terms Related to Home Water Systems. Centers for Disease Control

  2. Sewer Division. City of St. Clair Shores, Michigan.