If you have a garbage disposal you may have noticed it backing up into other sinks. This can cause foul smells, dangerous germs, and nasty stains. Thankfully it’s easy to diagnose and fix the issue.

A blocked drain or an incorrectly installed Garbage Disposal are the causes of garbage disposals backing up into other sinks. A P-trap or T connection will force wastewater upwards to the other sink. Poorly installed wastage pipes will cause slow drainage and water backup.

This article will cover why this is happening as well as provide you with practical, step-by-step solutions. So, let’s get right into it.

2 causes of Garbage Disposal Backing Up Into Other Sinks

When your garbage disposal backs up into other sinks, this is normally caused by a blockage. However, the root cause of the blockage varies based on your particular setup. I’ll run through the three most common causes and how to address them. 

Note: I’ve assumed here that your sink has a properly vented system, as all should, but read my other article if you’re unsure: Does a Sink Need a Vent (Explained). If you do not have an air vent this could be the cause of your garbage disposal backing up into the other sink.

1. Clogs In the Drain Pipes   

A clog in your sink drain pipes is the most likely cause of your garbage disposal backing up. Many people assume that their sink is not blocked because the water flow hasn’t stopped entirely. However, a small blockage can easily turn your pipe into a clogged vent and cause your garbage disposal to back up.  

Your two sinks are plumbed together in a T or Y formation and there’s probably a blockage downstream from them (or in the T itself, which is a common place for blockages to form).

When the disposal is turned on and pumps waste downstream, the blockage limits the flow and the water pressure sends wastewater back up towards the other sink. 

Initially, you may notice water draining slowly in addition to your garbage disposal backing up into the other sink. I once had a partial blockage for over a month, but didn’t realize it until the situation got worse and worse. Blockages only get worse so it’s best to address the issue ASAP.

Although this sounds complex, I’d definitely recommend you give this a go rather than calling a plumber (who will cost at least $100 just for a call-out charge). 

The Solution  

As a first step let’s try and clear your blockage through the traditional plunger: 

1. Fill the sink with water, up to 3 or 4 inches.
2. Plug the sinkhole of the garbage disposal.
3. Use the plunger in an up-and-down motion to create suction in the other sink. 
4. Open the sinkhole you covered earlier and start the faucet to clear the clogs.
5. If the clog isn’t cleared, repeat the procedure and, this time, use the plunger for a longer interval.

If the plunger doesn’t work, the blockage is probably quite far down your waste pipes. I’ve seen loads of grease blockages further down the system due to the combination of hot water and grease. Hot water carries the grease further down before solidifying. Try this: 

• Flush the sink with boiling water and dish soap. This may free up grease. 
• Pour a small amount of baking soda and then a cup of vinegar down your sink.
• As a last resort, you may need to remove the P-trap and manually alleviate the blockage. This can be a bit tricky but here’s a useful video that I found: 

Once you’ve cleared the blockage, re-try the garbage disposal. Hopefully, this has fixed the issue but if not move on to step 2. 

2. Faulty Installation   

If the sink next to your garbage disposal is backing up following a new installation, there’s a good chance the installation is incorrect. Even though you’ve ensured there are no blockages, incorrect installation could cause water to back up. 

One (sloppy) installation fault I regularly see is where the P-trap is too large for the system. Another example is where the drain outlet through the wall sits above the garbage disposal waste, therefore, working against gravity. This would cause your disposal to always hold waste. 

This often happens when a garbage disposal unit is installed beneath older kitchen countertops which are usually shorter. They weren’t designed for modern-day garburators.  

The last simple issue could relate to your tubular T. The diverter in the ‘T’ part of your system should be designed for disposal use – you should be able to check this by looking up the part number.

The Solution  

The picture below shows how garburator waste pipes should look. Note how the P-Trap is underneath the garbage disposal and also how the drain outlet through the wall is on a lower level than the garbage disposal too.

Compare this with your set-up – do you notice if the P-trap is significantly larger than this set-up or if the wall outlet is above the garbage disposal?

If your plumbing is incorrect, you could either give it a go yourself or phone a plumber to redo your drainage system. A flexible P-trap kit like this one (on Amazon) would be a simple way to address obvious plumbing faults. Recently there’s been a trend of flexible drainage pipes rather than the traditional plastic tubular type.

If you want to go the traditional route, here’s an excellent video on how to re-plumb drainage for a two-sink system (yours will be like this since you have a garburator backing up into the other sink):

How to Prevent Garbage Disposal From Backing Up  

Hopefully, I’ve helped solve your problem but let’s ensure it doesn’t happen again by taking precautions.

The general guidelines to prevent garbage disposal from clogging or backing up into other sinks are as follows:

• Try to keep the oil, grease, food scraps, and foreign objects far away from the drains.
• Always monitor the installation procedure thoroughly. Ensure that the plumber never wrongly installs the height of the drain and disposal outlets. 
• Do not try to save space by complicating the pipe network into a single space. These whirls and twirls can cost you a lot if they get clogged. 
• Maintain the pipes regularly.
• Use de-greasers (on Amazon) while weekly cleaning to eliminate fats already built up inside. Never use harsh chemicals for cleaning as they damage the plumbing environment. 
• Clean the pipes only if you have enough experience. It isn’t just a typical cleaning procedure. Rather it involves a certain level of expertise. That is why it would be better to call a professional for the job.

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