How to Know If Your Home’s Main Sewer Line Is Clogged

There’s something weird going on around your home lately. Your sinks seem backed up, as do your other plumbing fixtures. You ran a small store-bought snake through the drain and even used some drain cleaner, but nada. This sounds like an issue with your main sewer line, but how can you be sure?

Here’s how you can confirm that your home’s main sewer line is clogged:

  • When you flush the toilet, the shower begins filling up
  • The sink fills when you take a shower or run the washing machine
  • Your toilet is gurgling and/or smells stinky
  • The floor drain or sewer cleanout are leaking sewage

Discovering a clog in your main sewer line is never a fun experience, as you can’t use your toilet, shower, or sink until it’s fixed. This guide will provide you with all the information you need to know about handling the sewer line backup, including what causes it, how to fix it, and even some ways to prevent future clogs. 

What Is Your Main Sewer Line and Where Do You Find It?

If you’re a new homeowner or you’ve owned your home for a few years but never had a main sewer line issue, your first question is going to be what is this line? Then you’re going to wonder where it is.

Let’s start by talking about your main sewer line first, then where to locate it.

Main Sewer Line 101: What You Need to Know

The main sewer line is also referred to as your mainline or, simply, your sewer line. It’s a series of plumbing pipes and fixtures that connect from your home’s septic tank to your city connection. The drains in your home, including in your tubs or showers, release into the mainline. 

A mainline is not the same as a drain line. Every toilet, shower, and sink has its own dedicated drain line. The confusing part is that since drain lines are sometimes connected in rooms like the bathroom, if one drain backs up, the others can as well. This sounds like a mainline issue but it isn’t.

The biggest difference between the drain line and the main sewer line is that the latter has a clean-out fitting, well, usually. Not all mainlines do, and that can be problematic when issues arise with your main sewer line.

The clean-out fitting allows for access to the main pipe. The plumber can insert a snake into the pipe since the angle of the clean-out fitting prevents the wastewater flow from being disrupted. If your main sewer line doesn’t have a clean-out fitting, then reaching the main pipe is a much more difficult process. You should expect a higher plumbing bill as a result.  

Finding Your Main Sewer Line

Okay, so you know what your main sewer line is now, but where is it located on your property? You’re looking for the sewer cleanout point, which is the exit point of the line. This pipe should have a diameter of about four inches. The top of the pipe will feature a screw cap with a knob or notch that’s shaped like a square. 

Whether the sewer cleanout point is indoors or outdoors depends on the home. We recommend starting indoors down your basement. If it’s not there, check your crawlspace if your home has one. Should you still not find the cleanout point, it may be in your garage. Otherwise, it’ll be somewhere in your yard.

A yard sewer line will frequently be in one of two spots. The first is nearest the curb so the sewer line can easily connect to the municipal sewer drain. The line could also have been installed near your home’s foundation. You won’t be able to miss it once you discover it, as parts of the line do protrude out of the ground. 

Well, most of the time. Some sewer cleanouts are buried, in which case finding it will be practically impossible. In some instances, homes are built without sewer cleanout points at all.

Your plumber will know where to access the main sewer line, so if you can’t find it yourself, try not to worry too much. 

4 Ways to Tell Whether Your Home’s Main Sewer Line Is Clogged

Since locating the main sewer line isn’t necessarily easy, you won’t be able to peer inside to tell if it’s backed up. Despite that, you shouldn’t be left guessing what’s wrong, as your plumbing fixtures will begin behaving very strangely. 

Here are 4 overt signs of a main sewer line clog. 

The Shower Fills up When You Flush

You’re about to take a nice, hot shower after a long day’s work. Before you do, you go to the bathroom. You flush the toilet like usual, wash your hands, and then pull the shower curtain back. The tub is already full of water.

You’re the first one to use the shower today, so it shouldn’t have any water in it. You check your shower drain, which is working fine. Before you turn on the shower faucets, you wait and watch to see what will happen to the water in the tub. 

It takes several minutes, but the water eventually drains. When you shower, water pools to your ankles, maybe even close to your knees because the water in the tub just isn’t draining. As you step out, it again takes minutes for the water to fully drain. 

The Sink Fills When You Use the Washer or Take a Shower

Perhaps you exit the shower to a different sight, a sink that’s full of water. You know you didn’t leave the sink full when you turned the shower on. Like you did with the shower, you check the sink drain, but it’s open, so water shouldn’t be in the sink bowl.

Maybe you’re in the bathroom after running your washing machine and you notice the sink is full. Either way, you know there’s a problem.  

Your Toilet Gurgles and/or Smells

Here’s a more unmistakable sign. You’re in your bathroom and greeted by the sound of your toilet gurgling. This noise will probably only start when you’re showering or washing your hands in the sink. 

That gurgling might be accompanied by a rancid smell, which is the odor of backed-up sewage.  

The Floor Drain or Sewer Cleanout Are Leaking Sewage

That may not be the only place where you spot sewage. Check your floor drain in the basement as well. Since the main sewer line is backed up, the wastewater and sewage that usually go in the pipes underground get stuck. The floor drain, which is supposed to suck up water, will now release filthy water.

The sewer cleanout point will do the same thing, so run down to your garage or basement or even to your yard to see what’s coming out. It won’t be pretty! 

What Causes Clogs in the Main Sewer Line?

You’re pretty certain you have a main sewer line clog, and you can’t help but ask why? Is it a big clump of hair from the shower that got stuck in there? Maybe it’s bathroom waste backup? What is it?

Actually, the most frustrating thing about main sewer line clogs is that they’re sometimes not caused by anything the homeowner did (or didn’t) do. 

If you have a tree in the front or backyard near the mainline, its roots can grow in the path of the pipe, even going through the pipe. That will clog the line in a jiffy. 

An old home will have equally old main sewer line pipes that can crack under the strain. If the pipe is brittle and snaps, the contents within the pipes can’t follow their usual trajectory. 

Given the diameter of the main sewer line–which is at least six inches–it takes a pretty significant backup to clog it. That said, you do want to watch some of your habits, which can contribute to the problem.

For one, if you regularly pour grease down your kitchen sink, discontinue doing that. The grease doesn’t stay liquid forever. Once it solidifies, if there’s enough in the pipes, the main sewer line can’t work as it should. 

That’s also the case if you flush things down the toilet that you’re not supposed to, including baby wipes and feminine hygiene products. Try to discourage your kids from dropping things in the toilet as well.

What to Do to Get Your Home’s Main Sewer Line Unclogged

Please don’t try a DIY plumbing fix on your main sewer line. It’s one thing to follow a YouTube plumbing tutorial for a drain line issue, but not your mainline. Call a plumber. They know the best way to reach your main sewer line and they have the tools needed to fix the problem.

Some plumbers can come same-day so you don’t have to wait long to be able to use your plumbing fixtures again. The plumber may unblock the line with an industrial drain snake or even a hydro jetter. These are the most basic fixes and thus the least expensive. 

If the work is more complicated, such as having to remove a tree root or replace the pipe because it’s old, brittle, or broken, then expect the job to cost more. A main sewer line repair at this level entails unearthing the pipe in some instances. 

Otherwise, the plumber will rely on highly pressurized water to blast through the pipe and push the clog out the other side. This isn’t necessarily cheaper, but it does save your yard from having to be dug up. 

Tips for Preventing Future Sewer Line Clogs

Since the main sewer pipes are on your property, you’re responsible for their preservation. The following tips should keep your mainline in better condition so it may not clog again. 

Get Your Pipes Cleaned Every Year

Keep the number of your plumber handy, as you’ll want to have them come out to your home at least once a year to clean the main sewer pipes with a rooter. This preventative maintenance can remove small obstructions before they become big ones that block up the line. 

Be Good to Your Toilet

We talked before about the importance of not throwing just anything down the toilet. For the most part, only toilet paper should go in there, and not in huge quantities either. Make sure your toilet paper is biodegradable so it disintegrates quickly. It’s the items that don’t break down that pose a risk to your main sewer line. 

Maintain Trees on Your Property

If any trees grow near your main sewer line now, keep an eye on them in the years to come. You may have to get the tree removed or at least maintained by a pro landscaper so its roots don’t grow out of control. 

Final Thoughts

A main sewer line clog can back up your sinks, toilet, and shower. In a worst-case scenario, your toilet can start gurgling and your floor drain will leak sewage. Now that you understand more about your main sewer line, how it works, and what to do if it gets backed up, you can keep this line clear! 


Geoff Southworth is the creator of, the site that helps new homeowners, investors, and homeowners-to-be successfully navigate the complex world of property ownership. Geoff is a real estate investor of 8 years has had experience as a manager of a debt-free, private real estate equity fund, as well as a Registered Nurse in Emergency Trauma and Cardiac Cath Lab Care. As a result, he has developed a unique “people first, business second” approach to real estate.

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