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Plumbing in the Home/Kitchen sink - What makes the first picture incorrect?



I was reading online and found this link:,525337

The first picture shows a kitchen sink connected upstream of the toilet.  Someone suggested that the kitchen sink should be connected downstream after toilet and the wet vent.  What made connecting the kitchen sink upstream of the toilet not a good idea?


ANSWER: Hi Thomas,
There is  no difference which side of the toilet you  connect to. The  issue is the venting . Venting and pipe sizes are determined by the distance away from the main stack and the number of fixture units tied into the same line. At the risk of boring you with design parameters, In most cases a 2" line to the sink and an 1 1/2" vent tied into the stack or run separately will work just fine. 2" will handle a disposal, dishwasher and any size residential double or triple bowl sink with ease. Most codes require that the vent , if run independently, must be increased in size prior to penetrating the roof, usually 2" or up one size to 3"


---------- FOLLOW-UP ----------

QUESTION: Hi Jay, thanks for the response.

In the picture posted in that link, all traps were being vented properly through 2" sized vents -- the toilet and tub was wet vented through the lav, and the kitchen sink had it's own vent.  Where would the vent issue be coming from?  

If there is no difference which side of the toilet it is connected and venting is proper, why was it suggested to move the kitchen sink after the 3x2 Y?


My guess is someone is trying to fix something that isn't broken. There is no definitive or practical reason dictated by standard    pluming codes or requirements that I know of here in NY state. If this is a reason it would have to be a  regional requirement but  in my experience I can see problems in some installation wher  space between joists would not allow piping this way.  

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Jay Nicholas


Plumbing & Heating (warm air, hot water & steam heat)


40 years in the business, 40 years as a lic. master plumber in NY State. Retired

Graduated Magna cum laude at the School of Hard Knocks

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Commercial, residential and light industrial. You name it ...I have probably worked on it

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