01In short
A sewer backup happens when wastewater can't leave the building fast enough, so it comes back up through the lowest drain. There are two very different reasons:
Roots, grease, a collapsed pipe or a clogged house trap block the building sewer. It happens in any weather, usually when someone uses water.
During intense rain, sewers (in many NYC areas, combined sewers carrying rain and wastewater) can fill and push water back toward buildings. It happens during or right after a storm and often hits several neighbors.
- Drain / waste / sewer
- Vent
- 1Intense rain fills the city sewer; many NYC areas have combined sewers carrying rain and wastewater together
- 2Sewage can rise back up the building sewer toward the house
- 3It comes out at the lowest opening first: basement floor drains, toilets or showers
- 4Backwater valve: a one-way device that closes against reverse flow if it is installed, accessible and maintained
- 5Fixtures above the backup level are usually unaffected, but avoid using them while a backwater valve is closed
Schematic, not to scale. Your building's layout may differ.
02Common symptoms
- Gray or brown water rising from a basement floor drain, shower or toilet
- Gurgling from drains when another fixture is used
- Several fixtures draining slowly at the same time
- Sewage smell in the basement, especially after rain
- Water appearing around a basement cleanout cap
03Sewer backup, plumbing leak or groundwater?
| What you see | Sewer backup | Plumbing (supply) leak | Groundwater / surface water |
|---|---|---|---|
| Water color & smell | Gray or brown; sewage odor | Clear; no odor | Clear to muddy; earthy smell |
| Where it appears | Floor drain, basement toilet or shower, cleanout | Under pipes, a fixture, the water heater, or the ceiling | Along wall–floor joints, through walls or the slab |
| When | When water is used upstairs, or during heavy rain | Constant, or when a fixture runs | During and after rain or snowmelt |
| Who usually handles it | Plumber (blockage); 311/DEP if the city main | Plumber | Waterproofing or drainage contractor; sometimes a plumber for sump pumps |
Not sure? See water in the basement for a longer triage guide.
04Possible causes
- Tree roots
- Roots grow into joints in older cast-iron or clay sewer pipes. Common on tree-lined blocks and in the suburbs.
- Grease & wipes
- Fats harden in the line; "flushable" wipes don't break down like paper.
- Clogged house trap
- Many older NYC houses have a U-shaped house trap on the building drain. Debris collects there.
- Damaged pipe
- Cracked, collapsed, sagging ("bellied") or offset sections, sometimes after nearby excavation.
- City sewer surcharge
- Intense rainfall exceeding sewer capacity. NYC DEP estimates about 59,000 one- and two-family homes are in areas vulnerable to backups.
- Blocked city main
- Less common; usually affects several buildings. Report it to 311.
05What you can safely check
- Is it just you? Ask neighbors. Several buildings affected during rain points to the city sewer.
- Which fixtures? Only the lowest drains suggests the main line; one fixture suggests a local clog.
- Weather. Note whether it started during heavy rain.
- Photos. Photograph the water, its level and any damage for insurance before clean-up.
- Cleanout location. Note where your cleanout or house trap is so the plumber can find it quickly. Don't open a cleanout cap during a backup: it can release a flood of sewage.
06When professional help is needed
Call a plumber when sewage is coming into the building, when backups repeat, or when multiple fixtures are affected. A plumber may clear the line (cable or jetting), run a camera to find the cause, and recommend repair, replacement or a backwater valve where suitable. In NYC, sewer repairs and backwater valves are permitted work performed by or under a Licensed Master Plumber.
07Local considerations
New York City. Backups are recorded through 311 and resolved by DEP when the city main is involved. The count rises in heavy-rain years:
What it shows: Backups track heavy-rain years. FY2022 includes Hurricane Ida (September 2021); the report gives that year only as "nearly 5,000", so the bar is shown as approximate. FY2024 rose about 32% over FY2023.
Counts complaints resolved, not every backup that occurred. Many backups are never reported to 311.
Table view
| Fiscal year | Complaints resolved |
|---|---|
| FY2022 | nearly 5,000 |
| FY2023 | 2,164 |
| FY2024 | 2,851 |
Hurricane Ida in September 2021 killed 13 New Yorkers, 11 of them in basement apartments, according to City Council testimony. If you live in a basement unit, know your exit route and keep valuables off the floor during flash-flood warnings.
New Jersey. Older cities such as Hoboken, Jersey City, Newark and Paterson have combined sewer systems in parts of town, so storm backups follow a similar pattern. Responsibility for laterals varies by municipality and sewer utility; check with yours.
Long Island & outer suburbs. Many homes, especially in Suffolk, use cesspools or septic systems. A "backup" there may mean a full or failing system, which calls for a different specialist.
08Frequently asked questions
Why does sewage come up through my basement floor drain?
Wastewater always finds the lowest opening. If the line from your building to the street is blocked, or the city sewer is full during heavy rain, sewage backs up the pipe and out of the lowest drain, which is often the basement floor drain, a basement toilet or a shower.
Is a sewer backup my responsibility or the city's?
In New York City, the property owner is generally responsible for the building sewer, the pipe from the building to the city main, including the part under the sidewalk. DEP is responsible for the main in the street. If you suspect the city sewer, report it to 311 so DEP can check the main. A plumber can tell you whether the blockage is in your line.
Will a backwater valve prevent this?
A backwater valve can stop sewage from flowing back into the building when it is installed correctly, accessible and maintained. NYC DEP's own review notes that backwater valves "can work, but have significant drawbacks". For example, while the valve is closed, water used inside the building has nowhere to go. Whether one suits your building is a question for a licensed plumber, and installation in NYC is permitted work.
Does homeowners insurance cover sewer backup?
Often not by default. Many policies exclude sewer or drain backup unless you add a specific endorsement. Check your policy, and photograph the damage before clean-up.
Is it safe to clean up a sewage backup myself?
Sewage carries bacteria and other contaminants. Keep children and pets away, avoid skin contact, wear gloves and boots, and ventilate. If water has reached outlets or appliances, don't enter until the power is off. Large backups, or any affecting drywall, insulation or carpet, are usually a job for a remediation specialist.
09Related
Sources & review
Last reviewed . General information, not legal or engineering advice. Rules change and individual buildings differ.
- NYC DEP, Sewer Backup report (Local Law 1 of 2023) — backwater valve assessment; vulnerable homes estimate
- City Limits, sewer backup complaints per the Mayor's Management Report
- Bronx Community Board 11, DEP responsibility summary (2023)
- NYLPI testimony to NYC Council Committee on Environmental Protection