How to Run Drain and Vent Lines

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.

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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 may 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 may 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.
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What You Need

  • PVC saw or power saw
  • Level
  • PVC pipe and fittings to meet codes
  • PVC primer and cement
  • Pipe strap
  • Fitting to join to existing drainpipe
  • Basic carpentry tools 

Before You Begin: Frame for a Toilet Bend

Before you begin, check your plumbing setup. A joist may be in the way of a toilet bend. 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.

How to Clean a Toilet

First You'll Need to Run the Main Drain Line:

Step 1: Align the Main Drain Line

Start with a length of 3-inch pipe long enough to reach the basement or crawlspace. You may be able to cut it to 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.

Step 2: 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.

Step 3: 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.

Step 4: 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.

Step 5: 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.

Step 6: 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.

Then, Run the Individual Drain Lines:

Step 1: 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.

Step 2: 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 may 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.

Step 3: 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.

Finally, Install the Vents:

Step 1: Level and Drill

Your codes may 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.

Step 2: 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.

Step 3: 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 4: 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 may 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.

More How-To Tips:

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.

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.

Tips for Unblocking a Shower Drain

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 may need to move the hole over a few inches. The top plate may be doubled, meaning you have to drill through 3 inches. You may need to drill with a hole saw first from below (shown), then from above.

Install a Bathroom Vent

How to Install a Roof Jack

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.

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.

You may 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.

Choosing 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: Paralell

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 may 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.

Other Drain Configurations: Horizontal Pipes

Your situation may call for another drain configuration. This example shows a single-floor home in which all the fixtures tie into horizontal pipes, which in turn run to the stack.

Bonus: How to Unclog a Drain

Other Drain Configurations: Two-Story

In this example for a two-story home, first-floor vent pipes run up to join the second-floor vents at the top of their runs, so that all the vents tie in at a point well above the second-floor fixtures.

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