Plumbing Vent Diagram

So with my experience and the long researches I have done, I will answer these questions and more in this article, so you need to read carefully and follow the instructions, see you there.

Understanding DWV lines

The term DWV commonly refers to drain, waste, and vent lines. Drain and waste pipes (we need not worry here about the difference between the two) are easy to understand: They carry liquid and solid waste out of the house.

Vent pipes are not as well known but are just as important. They provide air behind the flow of waste, so the waste can flow smoothly, without gurgling. Vent pipes also keep noxious gases from entering the home.


Somewhere in the basement, you will see at least one large-diameter (usually 4-in.) vertical drainpipe called the “main stack” or “soil stack.” Typically, drainpipes from two or more bathrooms in floors above-and other plumbing fixtures as well-feed into the main stack. The main stack runs down to a horizontal building drainpipe that usually runs under the concrete floor.

Most homes have more than one stack. Often there is a secondary stack, perhaps 2 in. or 3 in. in diameter, usually serving the kitchen. Secondary stacks also run into the building drainpipe under the basement floor.

Drainpipe Slope

A horizontal drainpipe must slope downward at least 1⁄4 in. per running foot. An easy way to test for this is to tape a 1⁄2-in.-thick block to one end of a 2-ft. level; when the bubble reads level, the slope is correct. A stronger slope is fine—even preferable—but a flatter slope at any point along the pipe’s path will lead to drainage problems.

Drainpipes And Vent Pipes

A number of “branch” drainpipes run from fixtures, either vertically or horizontally at a downward slope, into a stack. These pipes are often 11⁄2 in. or 2 in. in diameter. (Nowadays plumbers typically use 2-in. pipes.) A horizontal drainpipe must slope at a rate of at least 1⁄4 in. per running foot.

Vent pipes are needed to ensure that there will always be air behind the flowing waste. If there is no air, the waste will not flow smoothly and may get stuck in the pipes. Also, without proper venting, waste can siphon up into sinks and toilets. And vent pipes ensure against the entry of poisonous gases into your house.

Vent pipes must lead all the way up and out the house’s roof. In some cases, a single vent pipe runs from a fixture up through the roof, but more often vents from several fixtures tie into the main vent that runs out the roof.

Drain-Waste-Vent System

This overview shows common plumbing installations in a house. Here there are no vent pipes in the basement; in some newer homes, a vent pipe is provided.

If you are lucky, there may be an existing vent pipe nearby in the basement. Often the solution is to run a vent pipe up to the first floor and tie into an existing pipe there.

The Importance Of Venting

Unless local codes allow for the use of air admittance valves (AAVs), the first question you must ask when planning new plumbing service is: How will it be vented? Remember that a vent must run all the way up through the roof; starting in a basement, that can be difficult. “Wet venting,” using the main stack as a vent pipe even though it also acts as a drainpipe, is often not allowed.

Arcane and complicated rules apply to plumb vents, and you should consult with a plumber or your building department before running any vent lines. Perhaps the most important code is this: A horizontal vent pipe, which ties into a nearby vertical vent pipe, can, in most cases, be no longer than 5 ft. Therefore, any new services need to be pretty close to the existing vent pipe.


Drainpipes do get clogged from time to time, so codes require that there be cleanouts—places where you can unscrew a plug and insert a plumber’s rod—at certain points. If you remove a cleanout, be sure to install another one nearby. And if you install new pipes, be sure to include new cleanouts. Cleanout plugs must be accessible, which sometimes means installing a removable panel in a wall.

If you are lucky, there may be a vent pipe already installed in your basement. (That is often the case in newer homes.) If not, you may need to run a vent pipe up to the first floor and tie into an existing pipe there. Worst-case scenario: You have to run a new vent pipe all the way up through the ceiling.

Running a Vent Pipe Up

Running a vent pipe up through the basement ceiling and tying into a vent on the first floor is not complicated plumbing, but it can be a real pain to get at the pipes; often you must cut walls open, plumb, and then replace, patch, sand, and paint the drywall or plaster. If possible, position your new service so its vent pipe will be directly below the vent pipe above.

  • Open up the wall on the first floor to expose the vent pipe. Use a level or laser level to locate the spot on the first-floor bottom
  • framing plate that is directly above your basement vent pipe.
  • Drill a locator hole up through the plate (a long drill bit helps), then drill a large hole down from the first floor. Make a connection in the basement.
  • and tie into the first-floor vent pipe with a horizontal vent pipe.
  • In extreme cases, you may need to run a vent pipe all the way up and out the roof. In that case, you should probably hire a roofer to patch the hole, install rubber flashing around the vent pipe, and install roofing as needed.

An AVS To The Rescue, Maybe

All this venting trouble may be avoided if your local codes allow for the use of AAVs. Sometimes called mechanical vents, these handy little devices have internal flaps that open when the pipe is draining and close when the draining is finished. Though they are looked down upon by many plumbers and are not installed in new homes (which are vented entirely with pipes), they do have a proven track record.

Buy AAVs that fit your pipe size, or attach with reducer fittings.

In most cases, each AAV serves only one fixture and is installed near it. Because AAVs can fail, they should be accessible so you can change them if need be. Check with local plumbers and your building department to see if you can use AAVs, and, if so, exactly where they should be installed.

AAV placement

Air admittance valves may be placed on the trap arm inside the room or on the drainpipe. If they are installed on the drainpipe, there should be an access panel so you can change the AAV if needed.

Pipe Materials

Newer homes use only PVC (white plastic) pipes and fittings often referred to as “schedule 40 pipes.” It is not only inexpensive but also strong and nearly eternal. In an older home you may find black plastic pipe, acrylonitrile butadiene styrene, called ABS; if so, consult with your building department about tying into it with PVC.

You may find galvanized pipe for smaller stacks and drains, and castiron pipe for larger stacks and drains. Transition fittings are available for changing from older pipe materials to PVC. As can be seen in many of the photos in this chapter, cast-iron pipe, though strong and solid, can often corrode in time.

When transitioning from galvanized to PVC drainpipe, you could use flexible fittings like the one shown in photo 4b. Here, threaded transition fittings are used.

DWV Options


In this plan, a single vent pipe serves the toilet, sink, and shower drain. A 3-in. horizontal drainpipe runs through the middle from the toilet, and the sink and shower drain tie into it. The shower has an under-floor P trap; the sink has a P trap inside the room, and the toilet needs no trap.


Here, vent pipes travel up through the wall and over through basement ceiling joists. The drainpipes for the shower, sink, and toilet all tie in and enter the main drain at the same point.

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