How to Vent a Bathroom Drain System

Drain and waste vents serve two important functions: They allow sewer gases, which are concentrated in the drain lines, to escape and they allow air into the drain lines. The latter function is important because, without an air inlet, flowing water creates a vacuum that can slow it down, encouraging clogs to develop, as well as siphoning water out of the P-traps. While you could design your plumbing so that each drain had its own vent that extended through the roof, the usual procedure is to connect all the vents in a bathroom to one main pipe.

The main vent extends vertically through the roof.

Determine the location of the main vent stack in your house. If it runs through one of the walls in the bathroom you need to vent, you can probably tie into it. If not, you'll have to run a new stack through the roof to which you can tie the fixtures in the bathroom.

Draw a diagram. It will help you plan the layout, determine how much pipe and what fittings you need and avoid leaving one of the fixtures unvented by mistake. A vent should rise vertically from the point where each drain drops to the sewer before you route it to the main vent stack. Size the pipe for the main stack, if you have to install one, so it is wide enough not to become blocked by ice in the winter. Two-inch pipe is suitable for most localities.

Tie the main stack to the central fixture in the bathroom, usually the toilet, by gluing in a long-sweep tee at the point where the drain drops into the sewer and extending the vent line upward from there. Run it vertically through the roof. It should extend at least 12 inches beyond the roof line. Use flashing to seal the hole in the roof around the pipe.

Vent each fixture in the bathroom by installing a long-sweep tee at the point where its drain drops into the main waste line and gluing in a length of ABS pipe that is long enough to rise above the drain opening for that fixture. Route the pipe to the main stack along the most convenient path, either horizontally or at an angle that rises toward the stack. In some cases, it is convenient to tie two or more fixtures to a single pipe and run that pipe to the main stack.

Tie each horizontal vent to the main stack by gluing in a long-sweep tee for each one with the sweep facing downward. You cannot have any drains above the point where you tie into the main stack, so if your bathroom is on a lower floor, and you are tying into an existing stack, you have to extend the vents above the drains on the upper floor before you can tie into the main stack.

Things You Will Need

  • 2-inch ABS pipe
  • Abs drain fittings
  • ABS glue
  • Roof flashing


  • After you make a diagram of your proposed venting system, have it checked by a licensed plumber to be sure it doesn't include any code violations.
  • Some localities allow you to vent a sink by installing an in-line vent so you don't have to run pipes through the wall. Check with your local building department for the regulations concerning this type of venting if you plan on using it.


  • Avoid sharp changes in direction in your vent lines by using long sweep elbows. Debris can collect in sharp corners and block the vents.

About the Author

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.

Photo Credits

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