Why Is Bathroom Sink Backing Up Into Tub? How To Solve It?

bathroom sink backing up into tub

After a comfortable sleep, you went into your bathroom and saw water filling the bathtub without opening the tub faucet. You stand a while to figure out what happened, only to see with awe that the tub is backing up from the bathroom sink.

Of course, this incident might appear ghostly to freak your mind out of this world. So, why is the bathroom sink backing up into the tub? Also, why does the water back up in your favorite bathtub?

Water backing up from the bathroom sink and drain to the bathtub almost always suggest that the drain and main sewer line are clogged. This clog in the drain and sewerage line happens because of tree roots, debris, hair, age, and an outdated drainage system.

You need to remove the items that block the drain and sewerage line. For this, you may use a drain cleaner, a plumbing drill, and the traditional plunging system for the tub.

Wearing Signs of Clogged Drains for Water Backing Up Into the Bathtub

Water backing up in the bathtub and shower space is the worst incident you may experience. It is also the last stage of a clogged sewerage and drainage line. When the bathtub starts backing up the water instead of draining it, you have left with no choice but to go through a meticulous cleaning job. If you know the signs of a clogged drain, you can act quickly to avoid this condition.

You will see multiple signs when the drain starts getting blocked. It includes:

  • You may see foul odor coming through the bathroom sink and bathtub drainage. As the water takes time to pass through the drainage system, it releases a stinky odor.
  • There will be bubbling water in the tub and bathroom sink. As the sewerage line narrows, the water takes more time to pass through. Hence, you will see water bubbling in the tub and shower space.
  • The water won’t usually pass through when you remove the bathtub drain stopper. So, it will start backing up in the tub.
  • At worst, the water will stay in the bathtub or toilet pan when you flush the toilet after use.
  • Out of nowhere, the water will come into your bathtub. Usually, it is the least typical scene in the bathroom as this stagnant water came from the sewerage line. As a result, it will be smelly and unbearable.

These signs suggest a blocked sewerage and drainage line with 100% surety when these signs appear. But why it happens? Let’s find the answer.

Why Is Bathroom Sink Backing Up Into Tub?

The bathroom sink backing up in the tub isn’t standard and pleasing. But, it can freak out your heart and attack when it happens. Thus, you must find the reason for the water backing up in the bathtub as soon as possible and fix it.

1. Malfunctioning Bathroom Sink:

The bathroom sink is designed to work separately from the bathtub. However, at times, the sink and tub drainage may get mixed. When it happens, the high pressure and water flow of the sink will force the water to back up in the tub instead of going away.

This often happens in older homes where the drainage and water supply lines get mixed. So, you need to inspect the home water and drainage lines.

2. Tree Root in the Sewerage Line:

If there’s a big tree close to the sewerage line, its root may reach underneath. Thus, it will reach inside the sewerage line and block it. Many homes have large and old trees for beautification. Its root will damage the plastic sewerage lines to cause such a disturbance.

3. Hairs and Debris Inside the Drainage:

Due to high traffic bathtub and sink in the bathroom receives the most residues and hairs. As it accumulates inside the draining pipe, you will see clogs in it. The residual development happens because of soaps, grease, dust, etc.

When you don’t attend the sink and tub stopper for a long time, these residues and hairs go inside the drainpipe. It eventually blocks the line and forces water back into the tub.

4. Multipurpose Lines:

Many homes would use the same draining for the bathroom sink and bathtub. It happens because sink water has higher pressure and flow than the bathtub faucet. It happens because sink water has higher pressure and flow than the bathtub faucet. This water flow speed and pressure difference will force it to the tub.

How to Stop Bathroom Sink from Backing Up Into Tub

You must find and eliminate the reason fast when you see the sink water backing up in the tub. As we have already discussed the reasons for bathroom and tub draining blockage that cause the water to back up in the tub, let’s see ways to fix it.

1. Fix Any Leaks in the Sink Drain:

Not only clogs but also bathtub drain pipe leaking can be the reason for water coming back into the tub. So, you may use putty or plumbing tapes to fix smaller leaks. However, if the damage is too extensive, you must call for a plumber to fix it.

2. Use the Plunging Technique:

The plunging technique is often a system to unblock the draining pipe in the kitchen and bathroom. For this, block the sink drain with an old or used stopper. Then, apply a rag right on the overflow tube of your bathtub. Now, fill up the tub with at least 2 inches of water.

Also, fit the plunger on the tub drain. It will create an airtight seal. After waiting for a few minutes, lift the plunger. It will force a vacuum inside the drainage. The sudden lifting of the plunger from the drain will force the 2-inch water right through the drainage. So, it would remove all the blockage from the tub draining pipe.

You must follow a similar process to drain the bathroom sink to unclog it.

3. Use a Plumbing Auger:

A plumbing auger is a large piece of metal with a hook in its front or head. The hook looks like a snake, which gets the name snake auger. For example, you may remove the stopper from the sink drain and insert it to collect its residues and hair.

You need to keep moving the plumbing snake to collect the hair to see its effect. Alternatively, you can connect a bladder with a garden hose and insert it inside the drain. Now, fill it up with water and let it flow inside. It will ultimately force the water to go deep and into the sewage hole. The bladder floats inside the sink and tub drain for such a cleaning facility. However, it is a temporary solution, and you may need to repeat it after one to two months.

4. Apply Drain Cleaners:

Drain cleaners are made using acid and acidic elements. When you pour it inside the clogged drain, it will erode all the greasy residues and hairs inside the drain. So, with water, it can vanish. However, you need to use such acidic drain cleaners with caution.

The acidic cleaner will hurt if the pipe is made of cheap plastic or gets old. Thus, you may even experience leaks in the bathroom sink drain after applying the cleaners. It is not eco-friendly either.

Frequently Asked Questions

How do you unclog a main drain line?

Answer: You may use a drain cleaner or plunging mechanism to unclog the main drain line. Also, try not to put the residues and hairs inside the sewage line to help it remain workable.

What do you do when water backs in the bathtub?

Answer: Check the drainage for any possible blockage when water backs up in the tub. Also, check for sink and tub pipes getting mixed.

Can a tub and sink share the same drain?

Answer: The bathroom sink and bathtub will share an identical drain in most homes. It is a common practice to use similar draining for tubs and sinks, and it won’t hurt the sewage system too.

Can I snake a drain myself?

Answer: Yes, you can use a plumbing auger to snake the drain yourself. You need to insert the auger into the drain and move it to collect the residues as much as possible.

When I run the sink, the tub fills up?

Answer: When it happens, you must check the draining pipe. Residual development inside it will cause the tub to fill up with water from the sink.


The bathroom sink backing up into tub is the worst scenario. When it happens, you should run a video inspection and find the right cause. If small clogs are causing it, try cleaning the drain pipe to help the sink water go away. However, for tree roots blocking the main sewage line, you must call a professional to remount the sewage line.

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