Drain and vent lines are important aspects of your home's sewer system. We'll show you why they matter and how to install them yourself.

By BH&G Editors
Updated October 23, 2020
neutral tone nature wallpaper bathroom
Emily Followill

Installing drain and vent lines in your bathroom is a home project that can be completed in a weekend. Before you begin, have your plans approved by an inspector and consider the following:

  • Drain and vent lines must be positioned precisely, so you should install them before the supply pipes. It might be possible to simplify supply runs by moving a vent pipe over a few inches.
  • If you must run drainpipes across a floor, carefully calculate the amount of vertical space available; you must slope the drainpipe at 1/8 to 1/4 inch drop per foot.
  • Sometimes it's difficult to visualize how drainpipes will travel through walls and floors. Once you start assembling the pieces and testing them for fit, you might need to modify your plans.
  • Some inspectors prefer horizontal vent pipes to be sloped so condensation can run back to the drainpipes; others don't consider this important. To be on the safe side, slope the vents.

Check your plumbing setup first to ensure the project goes smoothly. You might find that a joist is in the way of a toilet bend, for example. If so, remove as much flooring as necessary to get at the framing. Cut the joist, install a blocking piece, and attach 2x4 cleats around the opening.

  • Working time 8 hrs
  • Start to finish 4 days
  • Difficulty Hard
  • Involves Carpentry Skills, Plumbing Skills

What you need


How to do it

Part 1

indirect connection toilet vent
parallel toilet vent
opposite side toilet vent
Step 1

Choose a Toilet Vent or Drain Configuration

Toilet Vent Option 1: Indirect Connection

If the toilet drain does not connect directly to a vent, you must find another way to vent it. If the drain line runs away from the wall where you want the vent, use a reducing Y and a 45-degree street elbow to point the vent line toward the wall. The horizontal vent pipe runs right next to the closet bend.

Toilet Vent Option 2: Parallel

If the vent wall is parallel to the drain pipe, install a 45-degree reducing Y and a street elbow to point toward the wall. You might need another elbow (of any degree) to position the vertical vent where you want it.

Toilet Vent Option 3: Opposite Side

If the vent wall is opposite the drain line, use a reducing Y and a street elbow. The fittings can be pointed straight at the wall or at an angle, as needed.

align the main drain line
Step 2

Align the Main Drain Line

Start with a length of 3-inch pipe long enough to reach the basement or crawlspace. You might be able to cut it to the exact length after it has been installed. Dry-fit a Y-fitting, a length of pipe, and a low-heel vent fitting as shown, aligning them precisely.

insert the assembly
Step 3

Insert the Assembly

Insert the assembly down through the wall plates and temporarily anchor it. Make sure the Y-fitting is low enough to allow for installation of the other drain lines. Once you are sure of the configuration, pull up the assembly and prime and glue the pieces.

place and secure
Step 4

Place and Secure

Place the assembly back into the hole. Secure the low-heel vent fitting to the framing with pipe strap. Secure the pipe from below as well.

align and glue
Step 5

Align and Glue

Dry-fit a length of 3-inch pipe and a 4x3 reducing closet bend to the low-heel fitting. Check that the center of the closet bend hole is the correct distance from the wall (in most cases, 12-1/2 inches from the framing to allow for 1/2-inch drywall). Check that the pipe slopes 1/8 to 1/4 inch down to the fitting. (If necessary, you can trim the top of the closet bend after the flooring has been replaced.) Once you are sure the toilet drain setup is correct, mark the alignment of the fittings and disassemble. Prime and glue the pieces. Support the closet bend with a strap.

run pipes
Step 6

Run Pipes

Run a horizontal pipe to the existing pipe and assemble the parts needed for tying into it. All fittings should be Ys or drain elbows so wastewater can flow easily. Hold the horizontal pipe so it's sloped at 1/8 to 1/4 inch per foot and mark the existing pipe for cutting.

assemble pipe fittings
Step 7

Assemble Fittings

If the existing pipe is cast-iron, take care to support it securely before cutting. In the setup shown, a 4x3 Y connects to the house drain using no-hub fittings (which should be used to connect to either cast iron or ABS). Once you are sure the fittings are correct and the horizontal pipe slopes correctly, make alignment marks. Disassemble the parts, apply primer and glue, and reassemble the pieces in order, starting at the existing drain.

Step 8

Mark and Cut

Slip lengths of 2-inch pipe down through the holes drilled in the floor plate for the tub and sink vents. Have a helper hold the pipes plumb as you mark the plate below for notching. Cut notches about an inch wider than the pipe to accommodate a fitting.

Editor's Tip
How to Run Vents Through the Ceiling

To run a vent pipe through the ceiling, first drill a test hole to make sure you won't bump into any joists in the attic. You might need to move the hole over a few inches. The top plate can be doubled, meaning you have to drill through 3 inches. You might need to drill with a hole saw first from below (shown), then from above.

Step 9

Dry-fit and Install

Cut and dry-fit the horizontal drainpipe and the fittings for connecting the tub and the sink drains. (A 3-inch horizontal pipe is shown, but your inspector might permit a 2-inch pipe.) Insert a street elbow into the Y and hold the other pieces in place to mark for cutting. Make sure the horizontal pipe slopes at a rate of 1/8 to 1/4 inch per running foot. Install a reducing tee and a 45-degree elbow (or street elbow if you need to save room) for both joints. If the pipe will be accessible, install a cleanout on the fitting for the tub; otherwise install a drain elbow instead of a tee.

Editor's Tip
How to Run a Shower Drain

A 1-1/2-inch drain trap is often permitted for a shower, but a 2-inch trap will ensure quick flow of water and will be less likely to clog. A shower has no waste-and-overflow assembly, so the rough plumbing consists of a cemented trap that rises to the correct height for the shower base.

Step 10

Align and Glue

To plumb the drain for the tub, dry-fit a 2-inch trap onto a length of 2-inch pipe that is longer than it needs to be. Study the directions for the tub to determine precisely where the trap should be located. Hold the trap-and-pipe assembly in place and mark it for a cut. Dry-fit and check that the horizontal pipe slopes correctly. Once all the parts are accurately assembled, draw alignment marks and prime and glue the pieces together.

Step 11

Level and Drill

Your plumbing codes might require the horizontal revent lines be as high as 54 inches above the finished floor, or at least 6 inches above the fixture flood level (the point where water will start to spill out). Use a carpenter's level to mark the studs for drilling holes. Run the horizontal vent lines sloped downward toward the fixtures at a rate of 1/8 to 1/4 inch per running foot. Drill holes, cut pipes, and connect them in a dry run using drain fittings.

Editor's Tip
How to Run a Vent Around An Obstruction

If a medicine cabinet, window, or other obstruction prevents you from running a vent straight up, you'll have to turn a corner for a short distance, then turn again to head upward. Horizontal runs should be at least 6 inches above the fixture flood level (the rim of a sink, for example).

Step 12

Install a Sanitary Tee

Install a sanitary tee facing into the room for the sink trap. The ideal height is usually 18 inches above the finished floor, but check your sink instructions to be sure. Cement a 1-1/4-inch trap adapter into the tee. Install a piece of 1x6 blocking and anchor the pipe with a strap.

connect to other pipes
Step 13

Connect to Other Pipes

In the attic, tap into a conveniently located vent pipe. Cut the pipe and connect a reducer tee fitting. Use no-hub fittings to connect a PVC fitting to cast-iron or ABS pipe.

Step 14

Check for Leaks

Run the new vent line over to the tee fitting. The pipes should slope gently away from the existing vent pipe so water can travel downward. Your inspector might want you to include a tee fitting to be used for testing: Once the drain system is assembled and cemented, plug the drainpipe at the lower end. Pour water into it until all the drain and vent pipes are filled with water. Allow the water to sit for a day to make sure there are no leaks.

roof jack
Step 15

Check Codes

Check local codes for the correct way to install a roof jack. In most areas, you will need to install a 4-inch pipe. Some areas allow for a plastic pipe to extend out the roof (shown); in other areas, a metal pipe is required. Purchase a roof jack with a rubber flange that will seal a 4-inch pipe.

Step 16

Cut Pipe and Hole

Cut the 4-inch pipe to roughly the same angle as the roof slope and hold it plumb, its top touching the attic ceiling. Mark for the hole, which will be oval. Cut the hole using a drill and a reciprocating saw.

Step 17

Anchor Jack

You might need to cut some roofing shingles back. Slip the jack under the roof shingles at its upper half; the lower half of the jack rests on top of shingles. Poke the vent pipe up through the rubber flange. To anchor the jack, lift up some shingles and drive roofing nails. If any nails are not covered by shingles, cover the heads with roofing cement.

Comments (1)

How difficult was this project?
October 5, 2018
Step one shows 3 vent configurations that all involve a horizontal piece. Then in the construction step it shows a different configuration that involves a low heel. This is confusing, also I'm trying to figure out of any of the first 3 configurations meet code, since code generally requires vents to be 45d or more when below the flood line of the fixture... I'm not a licensed plumber.

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