Septic tank backing up into the house: what to do

A septic backup happens when wastewater cannot get from your house into the tank, or out of the tank into the drain field, so it comes back up through the lowest drains in the house. Stop using all water right away, keep everyone away from the sewage, and call a septic professional. Most backups trace to one of five causes: a clog in the pipe to the tank, a blocked inlet baffle, a tank overdue for pumping, a saturated drain field after heavy rain, or a drain field that has failed.

What to do right now

1. Stop using water. No flushing, showers, laundry, dishwasher or running taps. Every gallon you send down the drain can come back up. Tell everyone in the house.
2. Turn off appliances that drain automatically, such as a washing machine mid-cycle, a water softener on a regeneration cycle, or an ice maker with a drain.
3. Keep people and pets out of the area. Sewage carries disease-causing germs (pathogens). Children and pets should not walk through it.
4. Avoid touching the sewage. If you must enter the area, wear long rubber gloves, rubber boots and eye protection or a face shield.
5. Turn off power to wet areas only if you can do it safely. If water is near outlets, appliances or a furnace, shut off the breaker from a dry spot. If the panel is in the flooded area, stay out and call the electric utility.
6. Check for a septic alarm. If you have a pump tank or aerobic unit and the alarm is sounding, see septic alarm going off.
7. Call a septic service provider or plumber. A backup with sewage on the floor is a same-day call.

Do not open the septic tank to look inside, and never lean into or climb into it. Tanks have very little oxygen and hold hydrogen sulfide, methane and other gases that can kill quickly. The EPA says only trained specialists should clean or repair tanks.

Is it a house clog or a septic problem?

Before you call, a few minutes of watching can tell you a lot. A clog inside the house usually affects one fixture or one bathroom. A septic or main-line problem affects the whole house, and it usually shows up first at the lowest drains.

What you notice	Likely problem	Who to call
One sink or one toilet is slow, everything else drains fine	Clog in that fixture’s drain	Plunger or plumber
One bathroom’s fixtures are slow, others are fine	Clog in that branch line	Plumber
Every drain is slow, worst on the lowest floor	Clog in the main line to the tank, or a septic problem	Plumber or septic pro
Toilet gurgles or bubbles when the washer or tub drains	Main line partly blocked, or a full tank or drain field	Septic pro
Water comes up in a basement floor drain or shower	Main line, tank or drain field	Septic pro
Slow drains plus soggy ground or odor over the drain field	Drain field failure or a tank overdue for pumping	Septic pro
Slow drains only after heavy rain	Saturated drain field	Septic pro if it does not clear
Slow drains plus an alarm sounding	Pump or float problem in the pump tank	Septic pro

Gurgling on its own deserves a closer look too. It means air is being forced through the pipes, which happens with partial clogs, a blocked roof vent, or a system that is close to full. The University of Minnesota notes that a blocked or frozen roof vent alone can cause gurgling and dry traps. If gurgling comes with sewer odors, our guide to septic smells walks through the vent and trap causes.

What causes a septic backup?

A clog in the pipe from the house to the tank

This is one of the most common causes and often one of the cheapest to fix. Wipes, paper towels, grease or a buildup of toilet paper can block the pipe. Tree roots or a crushed or sagging pipe can too. The Washington State Department of Health notes that the problem may show up first on the lowest floor, and a service provider can often clear it by snaking the line through a cleanout. Roots or a broken pipe usually mean replacing at least part of the pipe.

A clogged inlet baffle

A baffle is a wall or pipe fitting inside the tank at each end. The inlet baffle directs incoming sewage down into the tank and keeps the floating scum layer (grease and light solids) from plugging the inlet pipe. If the baffle is plugged with paper or solids, or if it has broken off, sewage cannot get into the tank. A septic pro can clear it from the tank opening, and replace it if it is damaged. Clearing the line without fixing a missing baffle usually means the clog comes back.

A full tank

When a tank is overdue for pumping, sludge and scum fill so much of it that solids reach the outlet or block the inlet. The EPA recommends pumping most household tanks every three to five years, and pumping sooner if the scum is within 6 inches of the outlet or the sludge is within 12 inches. If you do not know when your tank was last pumped, it is a likely suspect. Pumping relieves a full-tank backup, but a pro should also check whether solids have already moved into the drain field.

A clogged outlet baffle or effluent filter

Many tanks have an effluent filter at the outlet to keep solids out of the drain field. When it clogs, water backs up in the tank and then the house. Cleaning the filter is routine maintenance and a common fix.

Saturated soil after heavy rain or snowmelt

A drain field works by letting effluent (the liquid that leaves the tank) soak into the soil. When the soil is already full of water, the effluent has nowhere to go. The EPA advises cutting water use when the water table is high and not using the system while the soil is saturated. These backups often ease as the ground drains. If they come back with every storm, have the system checked for groundwater leaking into the tank and for drain field problems.

A failed drain field

If the drain field soil is clogged with solids or a biological mat, it stops accepting water even in dry weather. Signs include soggy or spongy ground, bright green grass over the field, odors, and repeat backups soon after pumping. This is the most expensive cause to fix. See drain field failure for the signs and options.

Pump or power problems

On systems with a pump tank, a failed pump, stuck float or long power outage lets the pump tank fill. Once the reserve space above the alarm is used up, plumbing in the house can back up. Texas A&M AgriLife Extension advises stopping all water use if drains start running slowly during a power outage, because that can mean the reserve is full.

Cleaning up after a septic backup

If sewage has come into the house, call your local health department for guidance on cleanup, as the EPA recommends. For heavy contamination, or any backup that soaked carpet, drywall or insulation, a professional water damage or sewage cleanup company is the safer choice.

If you clean up a small area yourself:

• Protect yourself. Wear long rubber gloves, boots and eye or face protection. Keep children, pets and anyone with a weak immune system away.
• Do not wash sewage down the drains. The EPA advises removing water without pouring it into sinks or toilets, since the system cannot take it yet.
• Remove soaked porous items. Carpet, padding, upholstered furniture and cardboard that soaked up sewage usually cannot be cleaned safely.
• Clean, then disinfect hard surfaces. Wash with soap and water first. The EPA suggests disinfecting floors with a chlorine solution of one-half cup of household bleach per gallon of water.
• Disinfect tools and clothing. The EPA recommends washing all tools, equipment and clothing used in cleanup, then disinfecting them with a mix of 90% water and 10% household bleach.
• Dry it out. Ventilate with fans and open windows. Do not use the area until it has been completely dry for at least 24 hours.
• Wash your hands thoroughly after cleanup and before eating. See a doctor if you develop stomach illness, fever, or an infected cut.

If you have a private well and sewage reached the ground outside, have the well water tested before drinking it.

When to call, and what repairs cost

Call a septic professional for any backup that affects more than one fixture, involves sewage on the floor, or comes back after you clear a clog. Call right away if a septic alarm is sounding or the ground over the system is wet and smelly.

Typical national costs for the most common fixes:

Fix	Typical cost
Septic tank pumping	$300 to $700
Septic baffle repair or replacement	$150 to $900
Effluent filter replacement	$200 to $300
Septic pump repair	$250 to $400
Average septic repair (all types)	About $627 to $3,041

Ranges are from HomeGuide and Angi, and local prices vary. See our septic pumping cost guide for more detail, or request quotes from local septic pros to compare prices in your area.

How to prevent the next backup

• Flush only human waste and toilet paper. Wipes, including “flushable” ones, paper towels, diapers and grease cause many line and baffle clogs.
• Pump on schedule. Most tanks need it every three to five years. Our pumping frequency calculator gives an estimate for your household.
• Spread out water use. Run laundry over the week instead of all in one day, and fix running toilets, which can waste up to 200 gallons a day.
• Keep rain off the system. Point roof drains and sump pump discharge away from the drain field.
• Get regular inspections. The EPA recommends an inspection at least every three years, and yearly for systems with pumps or float switches.