Water in Your Chicago Basement? Start Here.

Water in a Chicago basement comes in four ways: up the floor drain, over the rim of a sump or ejector pit, through wall cracks, or in at a window well or door. After a heavy storm it’s usually the drain, pushed up by a city sewer full of the block’s rain and sewage together. Call Chicago Plumbworks and we’ll set up a visit from a licensed plumber who works out how it got into your bungalow, two-flat or greystone and what it takes to keep it out. If it’s spilling out of a pit because of a pump that quit mid-storm, the visit starts at the pit.

Flooding in the basement: what do you do first?

Storm water or sewage: how do you tell?

In Chicago, water that comes up the floor drain during a storm is both. The sewer under the street is a combined sewer, one pipe for the block’s rain and the block’s toilets, so treat floor drain water as sewage even when it looks like rain. Where the water came in, and when, tells you which fix it needs.

The dry-day backup is the one that won’t go away by itself. A street sewer that’s over capacity drains down once the rain stops. Your lateral, the private 6-inch pipe out to the street, backs up any day once something’s lodged in it. The Metropolitan Water Reclamation District (MWRD) notes a clogged lateral can overflow in a storm even while the street sewer still has room. That points to trouble inside your own pipe: roots in the line at an old joint, a grease plug, or a section that sank.

Why do Chicago basements back up in heavy rain?

Because the street out front sits higher than your basement floor, and the sewer under it fills with rain and sewage at once.

Chicago decided in 1855 to raise its streets so sewers could run beneath them with enough slope to drain. Roads went up by roughly four to 14 feet in places, and owners jacked whole buildings up to meet the new grade. Basements were dug below it. In many bungalows and two-flats the floor drain is lower than the street and the manhole lids on it. Plumbing codes call for a backwater valve on any fixture set below the next manhole upstream, because a full sewer can reach it.

Under that street runs a combined sewer. Most of Chicago drains this way, and the City’s Department of Water Management maintains about 4,400 miles of sewer main. By the MWRD’s math, an 800-square-foot roof sends around 500 gallons into its lateral in a one-inch rain. The City says the storms hitting Chicago now can drop two inches or more in an hour on one neighborhood. The sewer fills, the water inside rises toward the street, and it leaves through the lowest opening it can reach. In a basement below the street, that’s the floor drain.

Isn’t the Deep Tunnel supposed to stop this?

It helps, at the far end of the system. The MWRD’s Tunnel and Reservoir Plan runs 110 miles of tunnels 150 to 300 feet underground to catch combined sewage before it overflows into the rivers. It holds more than 11 billion gallons today, rising to 17.5 billion once the McCook Reservoir is finished. But the MWRD’s own explainer says neighborhood backups often hit during short, intense storms while the tunnels are still filling. The squeeze is in the local sewer and the lateral between it and your house, so the protection has to be at the building.

Here’s that setup in cross-section, and the two ways a plumber can cut a basement off from it:

Why does the garden unit flood first in a two-flat?

Every unit in a two-flat or three-flat drains into one building drain and one lateral. When the street sewer backs up, or the lateral clogs, the water comes out at the lowest fixture on that line: the garden unit’s floor drain, shower or toilet. The flats upstairs look fine, but every shower and load of laundry they run adds to what’s coming up downstairs. When it starts pouring, text your tenants and ask them to hold off on laundry, dishes and long showers until the rain stops. The MWRD asks the same of everyone when it calls an Overflow Action Day.

Will a standpipe or backflow valve hold the water back?

Some of it. The floor drain is a trap in the slab tied into your lateral, there to take water off the floor, which also makes it the sewer’s way in. Each fix below blocks that path, and they hold back very different amounts.

Standpipes and floor drain plugs

A standpipe is a length of pipe threaded into the floor drain, open at the top. It raises the overflow point so a shallow backup stays in the pipe instead of on the floor, and it can stay in when the drain isn’t needed. FEMA’s basement guidance fits standpipes to basements that have historically taken on no more than a few inches. It says not to rely on a drain plug if you expect more than 3 or 4 inches, and that a float-type plug can be held open by a little grit.

Past that depth the pressure has to go somewhere. The Village of Palatine’s flooding guidance warns that a drain sealed tight under a surcharging sewer can break the pipe and heave the basement floor. A standpipe that lets water spill over the top is the safer way for it to fail. Treat both as stopgaps.

Backwater valves

A backwater valve is fitted in the lateral and closes when water starts moving back toward the house. Set in the right line, it keeps a full street sewer from pushing up your floor drain. Before one goes in, settle these:

• Access. Plumbing code requires the valve to be reachable, because it has to be opened and cleaned.
• What drains through it. Upstairs fixtures shouldn’t run through a basement valve. While it’s shut, nothing leaves the building through that line.
• Where the clog is. The valve stops water coming from the street. A blockage between the house and the street sewer still backs up behind it.

Some older setups have a manual gate valve instead, closed by hand when a storm starts, which only helps if someone’s home.

In a two-flat, the second point is the one to plan around. One valve on the main building drain closes for every unit at once, and while it’s shut the upstairs flats’ water has nowhere to go but the garden unit’s fixtures. The setup that avoids that, an overhead sewer, is the second drawing above.

Sewage water in the basement: what not to touch

After a sewage backup, treat everything the water touched as contaminated until it’s cleaned, and leave the appliances alone.

• The washer, dishwasher and basement toilet. Don’t run them until the line is clear. Every gallon goes to the same drain that’s backing up.
• A furnace or water heater that stood in the water. Don’t relight it. Gas valves and controls that went under can be damaged, and utility guidance says a water heater that sat in floodwater should be replaced. Ask about both on the visit.
• Drain cleaner. Don’t pour it into a drain that’s backing up. It won’t move a sewer surcharge, and whoever opens the line next has to work in it.
• Soaked cardboard, carpet and pad, upholstered furniture. The CDC says to throw out what can’t be cleaned or dried quickly, and the EPA lists carpet among the porous materials that may have to go.
• Cords and outlets near the water. The Red Cross says not to touch wet electrical equipment, or anything electrical while you’re standing in water.

How do you stop the next backup?

Cut down what your building sends into the lateral, then close the path back in.

Start with the roof. City code once required downspouts to tie into the house drain, so on an older building the roof may still empty straight into the sewer that’s flooding the basement. The City now asks owners to cut them off above the ground and run the water onto the lawn. The extension should end at least three feet from the foundation, pointed away from the neighbors. The City says the job doesn’t take a permit. Downspouts that feed an old catch basin come off the same way.

Then close the path. That’s the choice between a backwater valve and an overhead sewer, and the questions below are the ones a plumber asks first.

If the building has an old catch basin out back, it belongs in the same conversation, since a basin full of old grease slows everything the building sends to the street. Cleaning it out is part of stopping the next backup, along with the overhead conversion if you go that way.

Last, the pumps: a pit is only as good as its power, so plan backup power for every one.

What happens after the water is out?

After a sewer backup, cleanup runs on a clock. The EPA says wet materials dried within 24 to 48 hours usually don’t grow mold, and the CDC puts the same window on flood cleanup. Drywall that wicked water up from the floor and carpet pad that soaked through generally have to come out, since the wall and slab behind them can’t dry otherwise.

If the backup came on a dry day, have the line looked at while the cause is still in it. A camera run out to the street shows what’s in the pipe and how far out it sits.

In a two-flat, have the garden unit’s ejector pit looked at on the same visit, since debris from a backup can foul its float.

Clean water, one slow fixture or a broken sewer?

If the floor drain isn’t the problem, one of these probably is:

• A pipe split in a wall or ceiling and the water is clean, not sewage
• Only one sink, tub or toilet won’t drain while the floor drain stays dry
• The camera found a collapsed or broken sewer under the yard or parkway