Common Causes of Septic Backups and What Each One Means

Miami-Dade County notes that South Florida's groundwater is "usually closest to the surface at the end of the wet season (around September/October)," and a high water table is one of several things that can send water back up a shower drain in Miami-Dade and Broward homes. Knowing what causes a septic backup tells you whether you need a pump-out, a cleared line or a closer look at the system.

A septic backup happens when wastewater can't leave the house fast enough or can't leave the tank. The most common causes are a clog in the pipe between the house and the tank, a tank that is overdue for pumping, a blocked outlet baffle or filter, and a drain field that can't absorb water, which in South Florida is often made worse by high groundwater after heavy rain. Where the backup shows up usually points to the cause.

Start here: where is the water coming back up?

Which drains are affected narrows things down quickly.

1. A clog in the line between the house and the tank

A septic tank backing up into the house often has nothing to do with the tank itself. The main line carries everything from the house to the tank's inlet, and when it clogs, water backs up even though the tank may have plenty of room. Typical culprits are wipes (including "flushable" ones), hardened cooking grease, tree roots in pipe joints, and older pipe that has sagged or collapsed. More on this in our guide to whether a clogged pipe or a broken tank is behind your backup.

2. A tank that is overdue for pumping

Every septic tank collects sludge on the bottom and scum on top, and pumping removes both. When a tank goes too long between pump-outs, solids build up until they reach the inlet or outlet, leaving less room for wastewater and pushing solids toward the drain field. A full tank usually gives warning first: slow drains across the house, gurgling and odors near the tank. If that pattern sounds familiar, our septic tank pumping service page explains what happens during a pump-out, and this guide covers the warning signs that a tank needs pumping.

3. A blocked outlet baffle or effluent filter

Most tanks have a baffle or tee at the outlet that keeps floating scum in the tank, and many newer tanks also have an effluent filter. Both work by catching material, and over time that material can slow or stop the flow out. The tank then fills above its normal level even if it isn't due for pumping. With the lid open, a technician can usually see it: the liquid sits higher than the outlet pipe.

4. High groundwater and heavy rain

This cause matters more in South Florida than in most of the country. A conventional system depends on the soil under the drain field to absorb treated wastewater, and soil that is already full of water can't take more.

Miami-Dade County's Connect 2 Protect program explains that "in South Florida, our groundwater is just a few feet beneath the surface," that it rises during the wet season, and that low-lying areas affected by higher groundwater and flooding "may experience compromised septic systems." After a heavy storm, or late in the rainy season, a system that worked fine in spring can start backing up.

What helps while the ground is saturated:

• Cut water use as much as you can. The EPA's guidance for saturated systems is to relieve pressure on the system "by using it less."

• Don't have the tank pumped while the drain field is flooded. The EPA advises against it, because an emptied tank in saturated soil can shift or float and damage the pipes. Once water levels drop, a pump-out removes any silt that got in.

• Keep vehicles and heavy equipment off the drain field. Wet soil compacts easily, and compacted soil absorbs less water.

Source: U.S. EPA, Septic Systems: What to Do after the Flood and Miami-Dade County, Connect 2 Protect.

5. A drain field that can't absorb water anymore

If solids escape the tank for a long time, they can clog the gravel and soil in the drain field. Roots, vehicle traffic and long stretches of high groundwater also reduce how much water the field can take. The drain field is the likely cause when the tank has been pumped and the outlet is clear but water still backs up soon after, or when the ground over the field stays soggy or smells in dry weather. Our article on how a dirty tank leads to drain field failure walks through how this develops. A septic tank inspection is the way to find out what condition the field is in before deciding on next steps.

6. Too much water in too short a time

A tank needs time for solids to settle before water moves on to the drain field. Back-to-back laundry loads, a houseful of guests or a toilet that runs all day can push water through faster than the system can handle, and in a system that is already near full or fighting high groundwater, that can tip it into a backup. Spreading laundry across the week, fixing running toilets and pointing downspouts away from the tank and drain field all help.

7. Grease, chemicals and things that shouldn't go down the drain

The bacteria in a septic tank break down waste. Large amounts of grease, harsh drain cleaners, bleach and some additives upset that process, so solids build up faster, and grease also hardens in pipes and at the outlet. Wipes, diapers, coffee grounds, food scraps from a garbage disposal, paint and solvents all add to the load. See what not to flush for a fuller list.

8. Damaged pipes, a cracked tank or a failed pump

Physical damage can cause a backup with little warning. Pipes crack or separate when the ground settles or roots grow in, and a cracked tank or broken baffle can let groundwater in or solids out. Systems with an aerobic treatment unit or a tank set lower than the drain field rely on a pump and alarm, and if the pump fails, wastewater stops moving.

If your system has an alarm panel and it's sounding, or the tank is visibly damaged, it's a septic tank repair situation rather than a routine pump-out. Our guide to septic tank cracks explains what causes them.

Will pumping fix a septic backup?

Only if the tank is the problem. Pumping helps when the tank is full, when solids are building up at the outlet, or after a flood has pushed silt into the tank (once the soil has drained). It won't fix a clogged main line, a collapsed pipe, a failed pump or a drain field that can't absorb water, and the backup will come back.

A pump-out is still often a sensible first step, because an empty, open tank lets a technician see the inlet, outlet, baffles and liquid level. Ask for a plain explanation of what was found and why the fix matches it.

What to do when your septic system is backing up

1. Stop using water. Hold off on laundry, the dishwasher and showers, and limit flushing.

2. Check where the water is coming up, using the table at the top of this page.

3. Keep people and pets away from any sewage, and wear gloves if you need to move anything that got wet.

4. Don't open the tank yourself. Septic tanks can contain dangerous gases, and the EPA recommends leaving tank work to trained professionals.

5. Skip chemical drain cleaners. They rarely clear a septic blockage and can harm the bacteria in the tank.

6. Call a septic company and tell them which drains are affected, when the tank was last pumped, and whether it has rained heavily.

Questions to ask before you book

It's easy to agree to the first fix offered during a backup. These questions help you understand what you're paying for:

• What do you think is causing the backup, and how will you confirm it?

• Will you check the inlet, outlet baffle and filter while the tank is open?

• If pumping doesn't solve it, what is the next step you'd recommend?

• Is the drain field involved, and how can you tell?

• Can you describe what you found, so I have a record for future service?

Frequently asked questions

How does a septic tank get backed up?

A septic tank backs up when wastewater can't flow into it or out of it. Common causes are a clog in the main line from the house, a tank full of sludge and scum, a blocked outlet baffle or filter, a drain field that can't absorb water, and high groundwater after heavy rain.

What should I do if my septic tank is backing up?

Stop using water right away, check which drains are affected, keep people and pets away from any sewage, and call a septic company. Don't open the tank yourself or pour chemical drain cleaners into the system.

Can you shower if your septic tank is backed up?

It's best not to. Showering adds more water to a system that already can't move it, which can push wastewater up through the lowest drains in the house. Hold off on showers, laundry and the dishwasher until the cause is found and fixed.

Will pumping the tank fix a septic backup?

Pumping fixes a backup caused by a full tank or solids at the outlet. It won't fix a clogged main line, a broken pipe, a failed pump or a drain field that can't absorb water. A pump-out does let a technician inspect the tank and find the real cause.

Can heavy rain cause a septic backup?

Yes. When rain saturates the soil around the drain field, the soil can't absorb wastewater from the tank. In South Florida, groundwater is closest to the surface at the end of the wet season, around September and October, so backups are more likely after storms in that period.

Does homeowners insurance cover septic backups?

It depends on your policy. Some policies exclude septic and sewer backups unless you've added specific coverage. Check with your insurance agent, and keep any service records or technician notes in case you need to file a claim.

Septic backup help in Miami-Dade and Broward

Septic Masters services septic systems for homes and businesses across Miami-Dade and Broward County. If your drains are backing up, call us at (786) 282-8690 or request a quote online. Tell us which drains are affected and when the tank was last pumped, and we'll help you figure out whether you need a pump-out, a repair or an inspection. You can also read more about our septic tank pumping service.