Why Is There Sewage Backing Up Into My Basement?

Finding sewage backing up through a floor drain or basement fixture is one of the more alarming things that can happen in a home. It’s also more common than people realize, especially in older Denver neighborhoods with aging sewer infrastructure. Our Denver drain and sewer experts at Bell Plumbing, Heating, Cooling & Electrical field calls about this regularly, and while it always feels like an emergency (because it usually is one), the causes tend to fall into a handful of well-known categories.

Why your basement takes the hit: Your home’s plumbing relies on gravity to move waste downhill toward the city sewer main. When something blocks that flow anywhere downstream, wastewater has nowhere to go but back up through the lowest drain it can find. In most homes, that’s a basement floor drain, a basement toilet, or a laundry standpipe.

Here’s what’s likely causing yours.

A Clogged Main Sewer Line

Your home has one main line carrying wastewater from every fixture out to the city sewer connection. If that line clogs, wastewater backs up and looks for the nearest exit.

Signs pointing to a main line clog:

• Multiple fixtures draining slowly at once
• Gurgling sounds from drains when you run water elsewhere in the house
• Backups that happen specifically when you run the washing machine or flush an upstairs toilet

Tree Root Intrusion

This is extremely common in older parts of Denver, where sewer lines were installed decades before current pipe materials and root barriers were standard. Roots are drawn to the moisture inside sewer pipes and can work into small cracks or joints, eventually growing large enough to block the line or crack it entirely.

Got mature trees near your sewer lateral and recurring backups? Root intrusion is worth investigating early. Hydrojetting can often clear root growth from the line, though severe cases may need a section of pipe repaired or replaced.

A City Sewer Main Issue

Sometimes the problem isn’t on your property at all. If the municipal sewer main is blocked, damaged, or overwhelmed during heavy rain, wastewater can back up into multiple homes on the same line at once.

This is more likely if:

• Neighbors are dealing with the same issue around the same time
• Backups happen specifically during or right after heavy rainfall

If you suspect a city main issue, contact Denver’s water department directly in addition to having a plumber assess your own lateral line.

A Failed or Overwhelmed Sump Pump

If your basement has a sump pump handling groundwater or drainage, a failed pump or one that’s undersized for a heavy storm can let water back up into areas it’s meant to keep dry. This is technically a different issue from a sewage backup, but the two can look similar and sometimes happen together during major storms. Bell installs and services sump pumps for homeowners dealing with recurring basement water issues.

Pipe Damage or Collapse

Older clay or cast iron sewer lines, still common throughout parts of Denver, can crack, shift, or collapse over time due to soil movement, age, or nearby construction. A collapsed section blocks flow completely and requires locating and repairing that specific stretch of pipe.

If It’s Happening Right Now

• Stop using water in your home immediately — every flush and load of laundry adds more wastewater to a system that already can’t handle what’s there
• Avoid walking through standing sewage, since it carries serious health risks
• Ventilate the area if possible while you wait for help
• Call a professional plumber right away for drain cleaning or sewer line diagnostics

This isn’t a situation to troubleshoot yourself.

Preventing a Repeat

Once the immediate backup is resolved, a camera inspection of your sewer line can identify whether root intrusion, pipe damage, or something else caused the problem. Scheduling regular drain maintenance, especially with mature trees on your property, catches smaller issues before they turn into full backups.

Don’t Wait This Out. Call Bell.

A sewage backup isn’t something that resolves itself, and every hour it sits is another hour of potential damage to your basement and risk to your family’s health. Bell Plumbing, Heating, Cooling & Electrical has served Denver homeowners since 1926, and our team offers same-day service for exactly this kind of emergency.

Contact us the moment you spot a backup. The faster we get there, the less cleanup you’re dealing with.

Call (303) 731-5469