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Plumbing in the Home/Drain System for Double Sink, One Shallow


I am installing a garbage disposal on the same side as my waste pipe on an offset double sink. The shallower sink is on the waste pipe side. The dishwasher also drains into the disposal which is on that side of the sink.  The waste pipe is NOT under the sink, but sets off about 8" to the side of the cabinet.  Then it goes through the floor.  (This is a corner sink base cabinet.)     My question is, can I drain from the larger sink, into a baffle T under the disposal, or would it be better to have both sinks drain to a centered baffle T?  I would prefer the first method because the disposal is blocking a direct pathway to the waste pipe.    With the second method I am also concerned with the added length of horizontal pipe which would drain from the centered P-trap to the waste pipe.  Any ideas?  I have had several suggestions, but none of them seem very good.  One person suggested I make a Y on the waste pipe and connect the sinks using two separate P-traps --that doesn't sound right.  Another says I MUST drain from the disposal to a baffle T under the larger sink, then double back to the waste pipe.  I dismissed this because not only is the disposal in the way of the returning schedule 40 pipe, but the horizontal pipe would be quite long.  Is there a reason I cannot put the baffle T under the disposal?

Typ Kitchen waste
Typ Kitchen waste  

Hello Mark,
Working with the plumbing piping, both waste and water, under the kitchen sink can be challenging. There can be a lot going on under there.

With a double basin sink, a disposer, hot & cold water feeds, hookups for the dishwasher drain and water supply and oftentimes some type of water filtration and even instant hot water dispensers all jammed into a rather small space can make things pretty interesting. I myself have spent 2 to 3 hours getting this all looked up and running properly.

Additionally, with a corner sink, while the cabinet itself may be spacious, these typically have a single door which makes for a pretty small access. If, like me, you are 6'2" and 250 pounds, it's going to be a challenge.

I will include a picture of a typical waste piping installation for a kitchen sink with a disposer.

The biggest issue I have with your described installation is that the waste pipe exits through the floor. That particular setup is not allowed to code, at least not here in the West. It may be different in your location. The problem with this type of set up is that it lends itself to creating what is called an "S" trap due to lack of venting. When the water drains down a pipe, it can create a vacuum in the airspace above the draining water if there is no event. I would strongly suggest that you install an AAV (Air Admittance Valve) on a sanitary T rather than just using an elbow to turn down into the drain pipe.

I would also caution against implementing the "Y" configuration you mentioned and including two traps. This is basically against most plumbing codes that I am aware of. Only a single trap can be used on any drain line for the same appliance or fixture. "Double trapping" is not allowed.

There are a number of "best practices" that I would recommend:
1. The disposer should be mounted on the shallower of the two sink bays to keep it as high as possible in the system.

2. If this is a new disposer, make sure that you knock out the slug inside the dishwasher drain fitting. Not everyone has a dishwasher and the new disposer's come with a blocked drain fitting by default.

3. The dishwasher itself should drain through an "Airgap" fitting mounted on the rear ledge of the sink. This provides a physical airgap between the dishwasher drain and the actual sanitary waste, thus preventing an illegal cross connection.

4. As you will see in the illustration I am providing, the typical layout drains the non-disposer side sink bay into the top of the baffle T. The disposer will drain through the branch of the baffle T. This connection can be straight across horizontally or its height can be adjusted by using a disposer elbow the typically comes with the disposer itself. The lower outlet of the baffle T typically feeds into the inlet of the trap.

Hopefully, my description is understandable and along with the illustration, you will be able to work this out.

Good Luck

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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


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