What causes sewer backup, and how to tell which cause is yours

01 /What does sewer backup mean?

Sewer backup means wastewater flowing the wrong way: instead of leaving the building through the building drain and lateral, it comes back up through a drain inside. Insurers, municipalities and contractors all use the term for water or sewage that enters a home through its plumbing, as distinct from water that comes in through walls, windows or the ground surface.

Your drainage system works by gravity. Every sink, toilet, shower and appliance empties into branch drains that join a vertical stack, then a horizontal building drain under or along the basement floor, then the lateral that runs under the yard to the city main. As long as the whole route has room, water falls downhill and leaves. When something in that route closes it off, or when the main at the end is already full, water has nowhere to go but up.

A backup is not the same thing as a slow drain. A slow bathroom sink is a local problem in its own trap or branch. A backup involves the shared part of the system, which is why the water appears somewhere other than where it was used.

02 /Why does sewage come up in the basement first?

Sewage comes up in the basement because the basement floor drain is usually the lowest opening connected to the building drain. When the line downstream is blocked, water from every fixture in the house backs up in the pipe until it reaches the height of the lowest open drain, then spills out there. An upstairs toilet can be flushed normally while its contents appear in the basement.

Other low openings behave the same way: a basement shower, a basement toilet, a laundry tub, or a floor drain in the furnace room. In a home with a basement suite, the suite's fixtures are usually the first to overflow, even if the tenants used no water at all.

People often ask whether sewage can back up into a washing machine. It is uncommon for sewage to enter the drum, because the washer's drain hose discharges into a standpipe or laundry tub that sits well above the floor. What usually happens is the reverse: the washer pumps out a large volume of water quickly, that surge hits a partial restriction in the main line, and the water comes up at the floor drain. A backup that appears every time the washer drains is one of the clearest signs of a main-line problem. If the standpipe itself overflows, the laundry branch or the line just downstream of it is the likely restriction.

03 /What causes sewer backups on your side of the property line?

Most single-house backups start in the private building drain or lateral. These causes build up over time and usually show warning signs first, such as gurgling toilets and a slowing floor drain. The table ranks them by how often they come up in residential service calls, as a general pattern rather than a statistic.

Sags deserve a special mention because they are easy to miss. When soil under a lateral settles, a section of pipe dips below its designed grade. Water stays in the dip even when nobody is using the plumbing, and solids that would otherwise be carried to the main drop out and collect there. Cleaning restores flow, but the dip keeps refilling, which is why sagging lines tend to need cleaning on a regular cycle until the section is re-laid.

Each of these causes has its own long-term answer, covered in the main sewer line cleaning guide, the fats, oils and grease guide and the wipes guide. If you are not yet sure the main line is the problem, start with the main sewer line clog symptoms guide.

04 /Can the city sewer back up into your house?

Yes. A municipal sewer can back up into homes in two ways: the main itself can be blocked, or it can be surcharged, meaning it is carrying more water than it has room for. In both cases the water level in the main rises above the level of the service connections, and sewage flows backward up the laterals into the lowest fixtures of the homes connected to it.

• A blocked main can be caused by roots, grease from many properties, debris or a structural failure in the city pipe. It tends to affect homes upstream of the blockage on the same street, in any weather.
• A surcharged main is usually a wet-weather event. Heavy rain or rapid snowmelt pushes extra water into the sanitary system until it runs full and the level rises.
• A blocked city portion of the service connection looks like a private blockage, because it affects one house. A sewer cleaner's camera and distance counter are what show whether it sits on the city side of the property line.

Where the private portion of a lateral ends and the municipal portion begins varies by municipality. If you suspect a city-side problem, report it: in Calgary and many other Canadian cities that means calling 311. Keep any camera footage that shows where the blockage sat.

05 /Can rain or flooding cause a sewer backup?

Yes, and it is the most common reason several homes back up at once. Sanitary sewers are meant to carry wastewater, not rain, but rain and meltwater get in anyway. Engineers call this inflow and infiltration: inflow is water that enters through direct connections such as roof downspouts, foundation drains or leaky manhole lids, and infiltration is groundwater that seeps in through cracked pipe and open joints.

1. Water enters the sanitary system

   Downspouts and weeping tile that were connected to the sanitary sewer in older homes, cracked laterals in saturated soil, and manholes in flooded streets all add clean water to a pipe sized for wastewater.

2. The main runs full

   Once the main is full, water has nowhere to go but up the service connections. Low-lying homes and homes near the downstream end of an area are often hit first.

3. Sewage flows backward

   Without a backwater valve, the rising water reaches the lowest fixture in each connected home. With a working valve, the flap closes and holds it out.

4. Your own water adds to it

   Every load of laundry or shower during the storm adds to the water in your own drain. If your valve is closed, that water has nowhere to go but your basement.

Some older parts of Canadian cities still have combined sewers, which carry storm water and wastewater in one pipe by design. Homes on combined systems are more exposed to wet-weather backups, which is one reason municipalities in those areas promote backwater valves and downspout disconnection.

06 /How do you tell which cause is behind your backup?

You can usually narrow the cause down from a few observations before anyone opens the line. The most useful single test is simple: stop all water use and watch the floor drain. If the water level falls or stays put, the problem is almost certainly a blockage in your own line. If it keeps rising with every tap off, water is coming from outside the house, from the municipal sewer or from the ground.

Once the line is open, the evidence gets much better. What comes back on the cable, such as roots, grease, wipes or soil, and what a camera shows at the distance where the cable stopped, together usually identify the cause with confidence. Ask for both, and write them down.

07 /Why does my sewer keep backing up?

A sewer keeps backing up when the cleaning removed the blockage but not the cause. Roots regrow into the same leaking joint, a sag refills with solids, grease keeps arriving from the kitchen, and a municipal main keeps surcharging in the same kind of storm. Repeat backups are a sign to diagnose, not just to clean again.

1. Log each event: date, weather, which drains overflowed, what came back on the cable, and the distance reached.
2. Get a camera inspection if you have not had one since the first backup.
3. Match the fix to the cause: scheduled cleaning for roots and sags, habit changes for grease and wipes, a repair for structural defects, and a backwater valve for municipal surcharge.
4. Set an interval with the Sewer Maintenance Planner and work through the layered plan in how to prevent sewer backups.

If a backup is happening now, go to the first hour of a sewer backup or open the Sewer Backup Checklist. When you are ready to have the line opened and diagnosed, you can request quotes from local sewer cleaners.

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