Skip to main content
Call(866) 202-9655

Problem · Chicago & inner-ring suburbs

Why does my basement keep flooding?

Water in a Chicago basement usually comes from one of four places: the sewer, the ground, a failed pump, or a leak. Knowing which one changes what will actually help.

Call for plumbing help(866) 202-9655

Calls are answered by a referral service that connects you with a local plumbing provider. How this works

First: stay safe

Safety first Do not walk into water that may be in contact with outlets, appliances or the electrical panel. If sewage is backing up, stop using water anywhere in the building, keep children and pets away, and avoid skin contact. If you smell gas, leave and call the gas utility or 911 from outside.
Rainwater pooling at a street inlet beside brick apartment buildings after a storm
Illustration

Why Chicago basements are exposed

Most of Chicago — and many older suburbs connected to the regional system — is served by combined sewers that carry rainwater and wastewater in the same pipes to the Metropolitan Water Reclamation District (MWRD). During heavy rain, neighborhood sewers can fill faster than they drain. Water then pushes back up the building sewer and out of the lowest opening, often the basement floor drain.

MWRD notes that sewers in a neighborhood can back up into basements or streets even when there is extra room in its interceptor sewers and the Deep Tunnel system, because local sewers may lack capacity to carry water to them.

Source: Metropolitan Water Reclamation District — Understanding Your Sewer

House sewer connection to a combined street sewer, with a backwater valveCross-section: a house with a basement floor drain connects through a private sewer lateral to a combined sewer under the street. A backwater valve on the lateral closes when water flows backward from the street sewer. GRADE BASEMENT floor drain access cover PROPERTY LINE (approx.) private side ←→ public way COMBINED SEWER rain + wastewater surcharge pushes back 1234
Diagram How a backwater valve protects a basement drain connected to a combined sewer.
  1. Basement floor drain — the lowest opening, so usually the first place sewer water appears.
  2. Private sewer lateral (building sewer) — typically the owner’s responsibility.
  3. Backwater valve — a flap that is meant to close when flow reverses; needs an access cover for cleaning and inspection.
  4. Combined street sewer — during heavy rain it can fill and back up toward buildings (MWRD).
Simplified schematic, not to scale. Actual layouts vary by building.

Four common sources

Common sources of basement water
SourceHow it usually shows upUsually needs
Sewer backupRises from floor drain, toilet or shower; gray or dark; sewage odor; during or after heavy rain, or when fixtures upstairs runPlumbing: drain/sewer inspection, backflow protection
GroundwaterSeeps through wall joints or floor cracks after long rainDrain tile/sump, or waterproofing (often not a plumber)
Sump failurePit overflowing; pump silent, cycling, or running nonstopPump repair/replacement, backup power
Plumbing leakSteady water regardless of weather; near pipes or the water heaterLeak repair

Use the interactive basement water guide.

Protections for sewer backup

MWRD describes check (backwater) valves and overhead sewers as valuable improvements that prevent sewer backups when properly installed, and advises cleaning backwater valves following manufacturer instructions.

Overhead sewer conversion with an ejector pumpCross-section: basement fixtures drain to a sealed ejector pit. The pump lifts wastewater up to a new overhead sewer line near the basement ceiling, which exits above the old floor-level connection. Upper-floor fixtures drain by gravity to the overhead line. BASEMENT upper-floor drains (gravity) sealed ejector pit WC old low connection (plugged) to street sewer 1234
Diagram An overhead sewer keeps the building's sewer connection above the basement floor so a backed-up street sewer cannot flow in through floor-level openings.
  1. Ejector pump with check valve lifts basement wastewater up; it needs power, so outages matter.
  2. Overhead sewer line runs near the basement ceiling.
  3. Building sewer exits the wall at a higher elevation.
  4. Former floor-level openings are sealed under the design.
Simplified schematic, not to scale. Actual layouts vary by building.
Backwater valve vs overhead sewer
AspectBackwater valveOverhead sewer
How it worksFlap closes when flow reversesBuilding sewer connection raised; basement fixtures pumped up
Needs powerNo (some designs add alarms)Yes — ejector pump
MaintenancePeriodic cleaning and inspectionPump and check valve maintenance
Usage during stormsUsing drains while closed can back up insideUpper floors drain normally
PermitTypically yes — confirm locallyTypically yes — confirm locally

Which is appropriate depends on the building, the sewer layout and local rules. Both are permit work in most municipalities.

Local programs and reporting

  • Chicago: report flooding to 311. We did not find a City-funded private backwater-valve program at our last review.
  • Oak Park: Sewer Backup Protection Grant — see Oak Park sewer backup protection.
  • Other suburbs (for example Des Plaines, Arlington Heights, Elmhurst, Morton Grove) have run cost-share or rebate programs; terms change, so confirm with the municipality.

When to call a professional

  • Sewage has come up through a drain or toilet.
  • Backups repeat in dry weather (likely a blockage or damaged line).
  • The sump pump is not running, or runs constantly.
  • You are considering a backwater valve or overhead sewer.

What affects the cost of a fix

Cause of the water; access (finished basements, concrete floors); building sewer depth and condition; whether a camera inspection is needed; permits; whether work extends to the public way; and pump power/backup choices. We do not publish prices.

Frequently asked questions

Why does my basement flood even though the City has the Deep Tunnel?

The regional tunnel and reservoir system reduces overflows, but the reclamation district (MWRD) notes that small neighborhood sewers can still fill and back up into basements even when the large interceptors and tunnels have room.

Is sewer backup the same as groundwater flooding?

No. Sewer backup comes up through drains and is contaminated. Groundwater seeps through walls or the floor, or is handled by a sump pump. They need different fixes. Use the basement water guide to compare the signs.

Who do I report street or sewer flooding to in Chicago?

MWRD advises reporting flooding to your municipality; in Chicago, that is 311.

Is there a City program that pays for a backwater valve?

As of our last review we did not find a City of Chicago program that funds private backwater valves; some aldermen have requested one. Several suburbs, such as Oak Park and Des Plaines, run their own grant or rebate programs. Check current terms with your municipality.

Related

Sources

  1. Metropolitan Water Reclamation District — Understanding Your Sewer Official / primary
  2. City of Chicago DOB — Blocking Rainwater and Preventing Sewer Backup Official / primary
  3. Village of Oak Park — Sewer Backup Protection Grant page Official / primary

Information can change; official sources control.

Call for plumbing help(866) 202-9655