Common Causes of Sewer Backups and How Plumbers Fix Them

A sewer backup sends wastewater the wrong way: up through floor drains, tubs and toilets instead of out to the city sewer. Early signs include slow drains in several fixtures, gurgling drains or toilets, and sewage smells. In South Bay neighborhoods built in the 1950s to 1970s, tree roots in clay sewer laterals are the usual culprit.

A backup is unsanitary and can damage floors and walls quickly, but most backups have a handful of causes and can be prevented. This post covers why backups happen and how each cause is fixed; if you’re trying to tell whether one is on the way, see 9 warning signs of a sewer backup.

Common Causes of Sewer Backups

Backups come from natural wear, what goes down the drains, aging pipe and, less often, problems in the public sewer. Knowing how the sewer system works helps you see where a problem starts: your lateral (the pipe from the house to the City main) or the main in the street. These are the ten causes we see most:

1. Tree root intrusion
2. Grease, oil and fat buildup
3. Cracked or collapsed pipes
4. Non-flushable items
5. Rainwater getting into the sewer
6. Combined sewer overflows (in older cities, not San Jose)
7. City sewer main problems
8. Aging pipe
9. Sump pumps and downspouts connected to the sewer
10. Wipes, hygiene products and paper towels

1. Tree Root Intrusion Into Sewer Lines

Roots seek moisture and find their way into sewer pipes through joints and cracks, especially in clay and cast-iron laterals. Once inside, they grow into a mat that catches everything flowing past. Early signs are slow drains, gurgling, repeated backups and sewer odors.

Left alone, roots can block the line completely and crack the pipe, and a backup can flood the house. A plumber cuts them out with a cable machine or hydro jetting, and may apply a foaming root treatment afterward to slow regrowth. Lasting fixes include lining the pipe or replacing the damaged section. See how to get rid of roots in a sewer line.

2. Fats, Oils and Grease (FOG) Buildup in Pipes

Fats, oils and grease go down the drain as liquids, then cool and harden on the pipe walls into a wax-like layer that keeps growing and catches food scraps. Kitchen lines clog first, followed by slow drains and eventually a backup.

Wipe pans and pour cooled grease into the trash rather than the sink. Enzyme drain treatments can help with routine maintenance, and a heavily coated line needs hydro jetting. Commercial kitchens use grease traps or interceptors to catch FOG before it reaches the sewer. If several fixtures drain slowly, those may be signs of a clogged main sewer line.

3. Cracked or Collapsed Sewer Pipes

Older clay, cast-iron and Orangeburg pipe becomes brittle with age. Ground settling, shifting soil, corrosion, roots and nearby construction crack it or knock joints out of line, creating offsets that catch debris and let roots and groundwater in. In the worst cases a section collapses and the line stops draining entirely.

A sewer camera inspection shows where the damage is and how bad it is. Depending on the pipe, a plumber can make a spot repair, line the pipe, use a trenchless method or replace the lateral.

4. Non-Flushable Items Blocking the System

Toilet paper is made to break apart in water; most other paper and fiber products aren’t. Baby wipes, paper towels, cotton swabs, dental floss and hygiene products stay intact, collect in the pipe and build into a soft clog that slowly restricts flow. Even wipes labeled “flushable” don’t break down fast enough to pass reliably through a home lateral. Put them in the trash. That protects your plumbing and the City’s sewer.

5. Rainwater Getting Into the Sanitary Sewer

In San Jose, storm drains and sanitary sewers are separate systems: storm drains carry runoff to creeks, and the sanitary sewer carries household wastewater to the treatment plant. Rain still finds its way into the sanitary side through cracked laterals and loose joints, and through downspouts or yard drains wrongly connected to it. In a heavy storm, that extra water can overload a lateral that’s already partly blocked and push sewage up through the lowest drain.

Signs include drains that back up only during or after rain. Repairing cracked pipe, disconnecting downspouts and drains from the sewer, and a backwater valve where the lowest fixtures sit near street level all reduce the risk.

6. Combined Sewer Overflow (CSO) Systems

Some older cities carry stormwater and sewage in the same pipes, a combined sewer system. In heavy rain the volume can exceed the system’s capacity, and untreated sewage can overflow into streets and waterways or back up into buildings. Those cities reduce overflows by separating storm and sewer lines and adding storage. San Francisco is the only coastal city in California with a combined sewer system.

San Jose’s systems are separate, so combined sewer overflows aren’t a cause of backups here; the local equivalent is rainwater leaking into the sanitary sewer (cause 5). If you own property in a city with a combined system, a backwater valve and clear drains help protect the building.

7. Municipal Sewer Line Backups

Less often, the blockage is in the City’s main: roots, grease, construction debris or a maintenance problem in the street. When the main backs up, wastewater can come up through the lowest drains of every house on the line, so neighbors with the same problem at the same time is a strong clue.

Report it right away. In San Jose, call the City’s 24/7 sewer line at (408) 794-1900; the City investigates at no charge, and when the problem is in the City’s part of the system, it clears the blockage and cleans up spills at no charge. A backwater valve on your line keeps a main backup from reaching fixtures below street level.

8. Aged or Deteriorating Pipe Infrastructure

Clay joints loosen, cast iron corrodes from the inside and Orangeburg (a tar-paper pipe used in some mid-century homes) deforms and collapses. As the pipe deteriorates, drains slow, leaks develop and, eventually, the line backs up. See types of plumbing pipes.

Regular sewer camera inspections catch wear early. Damaged sections can be repaired; for a lasting fix, the lateral is replaced with PVC or HDPE, which resist corrosion, roots and shifting soil better than older materials.

9. Improper Sump Pump or Downspout Connections

Sump pumps, downspouts and yard drains should discharge to the yard or the storm drain system, never into the sanitary sewer. Connected to the sewer, they add rainwater it isn’t sized for and cause backups in storms, and these connections aren’t allowed. Have them checked and redirect any that tie into the sewer, discharging away from the foundation; it’s part of good sewer line maintenance.

10. Flushing Wipes, Hygiene Products or Paper Towels

Wipes and paper towels are a leading cause of soft clogs in home sewer lines. They snag on roots, offsets and rough spots in older pipe, and each one catches the next until the line stops. Flush only toilet paper.

Clearing them takes the right tool: a plumber uses a cable machine (auger) or hydro jetting to remove the mass, then a camera to see whether roots or a pipe defect caught it. If the main line is fully blocked, see how to unclog a main sewer line, and call a plumber if it backs up again.

How Plumbers Fix a Sewer Backup

The fix depends on the cause, so finding it comes first.

1. Clear the line through the cleanout. A cable machine cuts through roots and wipes; hydro jetting scours away grease and fine roots.
2. Camera-inspect the line to find the cause and check the pipe’s condition; see sewer scope inspection.
3. Repair what’s broken: a spot repair, pipe lining or trenchless replacement for cracked, collapsed or corroded pipe. See sewer line replacement cost before deciding between repair and replacement.
4. Prevent a repeat: maintenance cleaning or root treatment where roots are a known problem, a backwater valve where fixtures sit near street level, and disconnecting downspouts or drains tied into the sewer.

If the City main is the problem, the City handles it (cause 7).

What to Do During a Backup

Act quickly: sewage carries bacteria and other harmful substances.

• Stop using water: no flushing, showers, laundry or dishwasher
• Keep people and pets away from sewage; stay out of water near outlets or appliances
• Don’t use chemical drain cleaners or open a full cleanout
• Call a plumber (CPI answers emergencies at (408) 978-3093), or the City at (408) 794-1900 if neighbors are backing up too
• Photograph the damage for insurance; see insurance for sewer lines
• Plan professional cleanup and disinfection for sewage on floors or carpet

How to Prevent Sewer Backups

• Keep grease, wipes and paper towels out of drains and toilets
• Schedule maintenance cleaning if roots are a known problem
• Camera-inspect older laterals every few years; CPI’s current free sewer line inspection covers homes with an accessible main cleanout
• Make sure downspouts and yard drains don’t connect to the sewer
• Consider a backwater valve if your lowest fixtures sit near street level

See signs of a clogged main sewer line, how to unclog a main sewer line and sewer line maintenance tips.

Sewer Backups in San Jose

• Roots in clay laterals under mature street trees are the leading cause in older South Bay neighborhoods.
• Separate storm drains: San Jose’s storm system doesn’t go to the sewer plant, but rain still finds its way into cracked laterals; see how the sewer system works.
• Holidays and storms put extra load on marginal lines.
• Sewer charges: San Jose single-family homes pay a flat sewer service charge on the property tax bill, so a backup or a repair doesn’t change it; see how to reduce sewer bills for what you can and can’t lower.

CPI Service clears and repairs sewer lines throughout San Jose and the South Bay, 24/7. See sewer line replacement, request a free estimate or call (408) 978-3093.