Septic backup: causes and what to do
If sewage is coming up into your drains, stop using water right away, keep people and pets away from the wastewater, and call a septic company. Most backups are caused by a full tank, a clog between the house and the tank, a failed pump, or a saturated drain field.
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What to do right now
- Stop using water. Don't flush, run the dishwasher or washer, or shower. Every gallon you use has to go somewhere, and right now that's back into the house.
- Keep everyone away from the wastewater, especially children and pets.
- Check for a pump alarm. If your system has a control panel with a light or buzzer, the pump may have failed or tripped its breaker.
- Call a septic company (or a plumber if you're not sure where the blockage is). Tell them whether the backup is in one drain or all of them.
- Protect yourself while cleaning: gloves, boots, and eye protection. Throw out soaked porous items such as rugs and cardboard, and disinfect hard surfaces.
Safety: Sewage contains disease-causing organisms. Don't let water from a backup reach electrical outlets or appliances, and shut off power to a flooded area only if you can do it without standing in water. Never open or enter the septic tank yourself.
One drain or the whole house?
Where the water shows up tells you a lot:
- Only one fixture is slow or backing up: the clog is probably in that fixture's drain line. A plumber can usually clear it.
- The lowest drains back up when you use water upstairs: the main line between the house and tank, or the tank itself, is blocked or full.
- Everything is slow and the yard is soggy or smells: the tank may be full or the drain field may be failing.
Common causes of a septic backup
The tank is overdue for pumping
When sludge and scum fill too much of the tank, flow slows and solids can block the outlet. This is the most common and least expensive cause to fix. See how often to pump a septic tank.
A clog in the main line
Wipes, paper towels, grease, and hygiene products can block the pipe between the house and the tank. Tree roots can also get into joints in older pipes.
A blocked inlet baffle or effluent filter
The inlet baffle can clog with solids, and an effluent filter on the outlet needs periodic cleaning. A clogged filter stops flow and backs water up, which is the filter doing its job. It still needs cleaning.
Pump or float failure
Systems with a pump tank rely on electric pumps and floats. If one fails, the alarm should sound, and wastewater can back up if water use continues.
A saturated or failing drain field
Heavy rain, a high water table, too much water use in a short time, or a field clogged by years of escaped solids can all prevent the drain field from accepting more water. This is the most serious cause and may need major repair or replacement.
Collapsed or damaged pipes
Vehicles driven over the line, settling soil, or old pipe materials can crush or separate the line.
How professionals diagnose and fix it
A septic company will usually find and open the tank and check the liquid level. If the level is low and water isn't reaching the tank, the problem is the line from the house, which can be cleared with a drain machine or jetter, often after a camera inspection. If the level is high, they'll pump the tank and then watch whether water flows back in from the drain field, which suggests a field problem.
Depending on what they find, the fix might be pumping, clearing a line, replacing a baffle or filter, repairing a pump or float, or evaluating the drain field. Ask for a written explanation of the cause and the recommended repair before major work. For big repairs, it's reasonable to get a second quote.
How to prevent the next backup
- Pump on a schedule based on your tank size and household.
- Flush only toilet paper. Put wipes, even "flushable" ones, in the trash.
- Keep grease and cooking oil out of drains.
- Spread out laundry and avoid huge water use in a single day.
- Keep downspouts and sump discharges away from the tank and drain field.
- Know where your tank is. If you don't, read how to find a septic tank on your property, and consider risers for easy access.
- Test the pump alarm occasionally if you have one.
Not sure whether you need a septic company or a plumber? If the backup involves every drain in the house, start with a septic company. If it's one fixture, start with a plumber. Our plumbing estimates page explains what plumbers typically handle.
Frequently asked questions
Will homeowners insurance cover a septic backup?
It depends on your policy. Many standard policies exclude sewer or septic backup unless you've added a specific endorsement. Call your insurer, document the damage with photos, and keep receipts.
Can heavy rain cause a septic backup?
Yes. Saturated soil can stop the drain field from absorbing effluent, so the water has nowhere to go. Cutting water use during and after heavy rain helps.
Is it safe to stay in the house after a sewage backup?
Sewage carries bacteria and viruses. Keep people, especially children, and pets out of affected areas until cleanup is done. Large backups or soaked drywall and carpet usually call for a professional cleanup company.