Short answer
The main sewer line is the large drain that carries wastewater from your house out to the city sewer or septic tank. To find it, look for a capped cleanout pipe near the foundation or the street, check the basement or crawlspace and trace where the largest drain leaves the house. Call 811 before any digging. In a backup, stop using water and call a licensed plumber or your utility.
Most people never think about the main sewer line until it backs up. Then the first questions are where the main sewer line is, who owns it and how to reach it. This guide shows how to find the main sewer line location from clues around your home, what the access point called a cleanout looks like and what to do if you see signs of a clog. It is written for US homeowners, with a note for Canadian readers.
What the main sewer line is
People use the phrase two ways, so it helps to separate them. Inside your house, the main drain collects every sink, tub, toilet and appliance. Outside, a pipe carries that wastewater to the public sewer or to a septic tank. The Buffalo Mountain Metro District in Colorado describes it this way: the house main drain runs under the home and connects to the house sewer outside, which joins the municipal sewer system. Where the two meet, code calls for a cleanout.
Cities and plumbers add a second term. The sewer main is the public pipe in the street. The line from your house to that public pipe is often called the lateral, the service line or the building sewer. The City of Fairfield, Ohio, for example, says the city is responsible for the public main and the homeowner is responsible for the lateral. The City of Sioux Falls says the same about its main and your private sewer line. Check the rule where you live.
If you have a septic system rather than a city sewer, the pipe from the house still leaves through the same kind of main drain, but it leads to a tank. See our signs your septic system is failing and septic maintenance near me.
How to find the main sewer line
You do not need tools. Work through these clues in order and stop when you find a capped pipe or a lid.
- Look for a cleanout. A cleanout is a capped access pipe on the main line. Buffalo Mountain says it can sit up to three feet beyond the home's outer wall, ideally with a lid labeled "Sewer Clean Out" in a concrete or plastic enclosure, though soil, mulch or plants often hide it. The West Valley Sanitation District in California describes a property line cleanout at the edge of private property whose surface box has a lid marked "Sewer."
- Check the street side. Buffalo Mountain suggests looking near the street for an "S" stamped in concrete or painted on the curb to mark the buried sewer line. The cleanout should sit along a straight line between that marker and the house.
- Look inside. The same district says homes may have other cleanouts, such as capped pipe stubs on exterior walls and access points in basements or crawlspaces. In a basement or crawlspace, look along the foundation wall for the largest drain pipe.
- Find where the line leaves the house. Follow the biggest drain pipe in the basement or crawlspace to the wall where it goes out. The cleanout is often near that point.
- Note the lowest drain. Floor drains and fixtures at the lowest level are where trouble often shows first. The City of Sioux Falls lists below-ground floor drains among the places to watch for blockage.
- Ask for records. Your city or sewer district, or your original house plans, may show where the line connects. Older homes can differ. Buffalo Mountain notes that older homes, code changes and sewers added after construction can put the cleanout somewhere unexpected. The City of Olympia, Washington says older homes often lack a cleanout at the property line.
Once you find it, take a photo and write down its location. If you cannot find it, a plumber can locate the line. For more on what a cleanout is for and how cleaning works, read sewer cleanout and sewer cleaning explained.
Main sewer line cleanout: do not open it casually
A cleanout gives a plumber direct access for cleaning or a camera. For a homeowner, the safe approach is to know where it is and to keep it clear and accessible. Leave opening it to the plumber unless a professional has told you how to do it safely for your system.
Ownership of the cleanout and the pipe around it varies. The City of Olympia says cleanouts there are owned by the property owner, and that the city owns the pipe from the main to a cleanout at the right-of-way line, if there is one. A utility near you may draw the line elsewhere.
Call 811 before any digging
Do not dig to find the line. Buried gas, electric, water and communications lines can be near the surface. The national 811 service, described at call811.com (which forwards to 811beforeyoudig.com), says it is free and that anyone planning to dig should contact it first, including for homeowner projects and shallow digging. It advises contacting 811 a few business days before you begin and having the address, nearest cross street, project type and exact digging area ready.
That is the US picture. Rules and lead times differ by state, so check your local 811 center. Do not assume the service will also mark a private sewer lateral on your property; ask when you call.
Canada. There is no 811 number to rely on. Locate services are set up by province, and the details differ. The Region of Durham in Ontario, for example, says contacting Ontario One Call before digging is the law there. Elsewhere, check with your municipality or provincial regulator before any digging.
Sewer line clog symptoms: signs of a main line problem
A single slow sink is usually a local clog. A main line problem touches more than one fixture. Look for these.
- Several drains slow at once, or gurgling in a drain when water runs elsewhere.
- Water not draining from below-ground floor drains, or water rising through floor drains, showers or toilets. The City of Sioux Falls lists these as signs of a sewer blockage.
- A toilet that bubbles when you run the washing machine.
- Backups that return after you clear them.
- A sewage smell or wet patch in the yard over the line.
Causes include grease, wipes, tree roots and damaged pipe. For roots specifically, see tree roots in a sewer line: signs and options. For a single clogged fixture, see clogged drain: what to try yourself and when to call a plumber.
What to do in a sewer backup
These steps follow the City of Fairfield, Ohio and the City of Sioux Falls, South Dakota. Local rules differ, so also check your own city.
- Stop using water. Fairfield says to stop all water use right away, including washing machines, dishwashers and showers.
- Work out where it is. Fairfield says that if only one appliance backs up, the clog is likely in that appliance's drain and a plumber can help. If water comes out of a basement floor drain, call your sewer utility.
- Tell your utility it is a sewer backup. Fairfield and Sioux Falls say city crews check whether the blockage is in the public main or in your private line. If it is in the main, the city clears it. If it is in your line, you hire a drain cleaning company.
- Call a licensed plumber for your private line. Ask for a camera look after the line is cleared, as Fairfield suggests, to find the cause.
- Call your local health department if sewage has come into the house or yard, and ask about safe cleanup.
Sioux Falls says raw sewage carries bacteria, viruses and fungi. It advises rubber boots, waterproof gloves and protective clothing during cleanup, washing hands afterward and avoiding the cleanup if your immune system is compromised. It also says surfaces should be cleaned and dried within 24 to 48 hours to prevent mold and that a professional cleaning service may be worth considering for a major cleanup. Keep records and photos for your insurance company.
If you smell gas, leave the building and call your gas utility or emergency number from outside.
What a camera inspection shows
A plumber feeds a camera on a cable through the cleanout and records the inside of the line. You can ask to see it live and to keep the video. It shows what is happening inside the pipe: the pipe material, where it bends, whether water sits in a low spot, roots, buildup and any cracks or collapses. Fairfield encourages this after a blockage is cleared, to find the cause and plan future maintenance.
The camera also helps you decide what to do next. If it shows grease, cleaning may be enough. If it shows roots or damage, ask about the options in our guide to tree roots in sewer lines. To compare local companies, use the method in drain cleaning near me: how to find and vet a company.
What to look up where you live
- Your sewer utility, sewer district or city public works: where the public pipe ends and yours begins, how to report a backup and whether records of your line exist.
- Your local 811 center: lead times and what it will mark, before any digging.
- State or provincial plumbing or contractor licensing board: to confirm a plumber's license.
- County health department or environmental health office: for sewage in the house or yard.
For Canadian readers. Provinces and territories regulate plumbing, and municipalities set their own sewer rules. Costs are in Canadian dollars. Use your provincial locate service rather than 811.
For plumbing, drain and septic company owners
Main sewer line location and sewer line clog symptoms are searches your customers make when something has gone wrong, often at night. If you want to be the company they reach, read emergency plumber marketing and drain cleaning marketing. You can also book a call with LiveLeadsLane. LiveLeadsLane is a marketing practice for contractors, not a directory, and it does not take jobs or refer homeowners.
