Excuse my noob question, but what does a vent stack look like? It's pretty easy to tell which pipes are for waste (I can hear the water running through them), and I'm assuming that all of the pipes with a cleanout are waste as well, but not sure what I should be looking for when trying to find a vent stack to tie into for my bar sink and dishwasher. Is it possible that my basement doesn't have any vent stacks since venting goes up??

1 Answer 1


There will always be a vent stack in a home if you have plumbing in it. they usually poke out the roof to vent and go all the way to the basement and under the basement floor. All the waste water dumps into these pipes and exits the house to the main waste pipe outside your home. Depending on how many bathrooms you have the number of vent stacks can differ but most homes with one bathroom and a kitchen have two vent stacks. The bathroom vent stack will be the larger one (3-4 inches) and the kitchen is generally around a 2 inch pipe I believe. Usually when you tie into a vent stack you cut out a chunk and insert a "y" fitting of some sort. I am no plumber by any means but I have observed this being done. Also, when you tie into a vent stack there is a code in my state where you have to have the vent for the tie-in go above the fixture above it (i.e. above where the kitchen sink drain dumps into the stack if you tie into that stack) I think the name of that would be a branch vent ( see my simple diagram below). Good luck.Simple vent diagram

  • Why would the vent stack connect to the sewer if it's for allowing air into the system / letting gasses escape? Commented Sep 12, 2013 at 13:36
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    That and you need a vent to allow the waste water to escape unobstructed. Think of it like dumping a 2 liter bottle upside down to empty it. It will chug and take a long time to empty. If you were to poke a hole in the bottom of the bottle and turn it upside down it would flow freely and a lot faster because it is drawing air into the bottle from the poked hole and not where the fluid is trying to escape. The stack going out the roof would be the hole in the bottle and your sink drain would be the bottle opening.
    – PMK
    Commented Sep 13, 2013 at 14:36
  • I think I understand your diagram better now. Commented Sep 13, 2013 at 17:48
  • The diagram is good. The purpose of the vent is to let air behind water flowing through your waste lines. This prevents that water from creating a suction (negative pressure) behind it and pulling water out of traps behind it as it flows. How to identify a vent? It shouldn't have water flowing through it that you can hear. If you have other fixtures already in your basement, look for pipe that goes UP from those fixtures. If nothing in your basement, look for pipes in your ceiling that are capped off. Could be a vent. If you don't find anything, you may need to run one all the way up. Commented Mar 8, 2016 at 18:49

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