Why Do Drains Back Up After Heavy Rain?

Southwest Florida has a wet season, and many residents have seen a drain that behaves all winter turn sluggish in July. A backup after a downpour can feel like bad luck, but there are usually understandable reasons. This post goes through the main ones. It is general information, not a diagnosis of any particular home.

The sewer system can take on extra water

A sanitary sewer is built to carry household wastewater, not rain. In practice, rainwater can get in. The US Environmental Protection Agency's page on sanitary sewer overflows lists leaky sewers, where stormwater and groundwater enter through cracks and joints, among the problems that can cause overflows. EPA also lists tree roots entering through defects, and fats, oils and grease and flushed products such as wipes, as causes of blockages (EPA: Sanitary Sewer Overflow (SSO) Frequent Questions).

Rainwater that enters a sewer is often called inflow and infiltration. When enough of it arrives at once, the system can be overloaded, and wastewater can come up at the lowest points, which may include the drains of a home. Whether that happens in a particular neighborhood depends on how the local system is built and maintained. This site cannot say which neighborhoods are affected, and we do not claim that any are.

Your own line can be part of the problem

A home's sewer lateral, the pipe between the house and the street connection, can have cracks, loose joints or root intrusion. In dry weather, those defects may do little. When the ground is saturated, groundwater can push into a loose joint and carry soil or roots with it, and the extra flow can make a partial blockage into a full one. EPA's own list of ways to reduce overflows includes repairing broken or leaking service lines, which are the pipes that connect homes to the main sewer.

That is one reason a camera look is sensible when a backup comes with the rain. It can show whether the pipe is simply dirty or damaged. See what a sewer camera inspection shows and the sewer line repair page.

Septic systems and saturated ground

If your home is on septic, rain matters for a different reason. A septic system sends water from the tank into a drain field, where the soil absorbs and treats it. When the soil is already full of water, it cannot take more. EPA's guidance for septic systems after flooding says that when the drain field is flooded or saturated, the best response is to plug basement drains and drastically reduce water use in the house, and that pumping the tank during these conditions is at best a temporary fix and, in the worst case, could cause the tank to try to float out of the ground and damage its pipes (EPA: Septic Systems, What to Do after the Flood).

The same page advises that the sewage system should not be used until the water in the soil absorption field is lower than the water level around the house. If you are on septic and drains slow down during a heavy rain, that is a reason to reduce water use and call a provider who works on septic systems. The Lehigh Acres, Cape Coral and North Fort Myers pages explain more about septic in the area.

Low-lying ground and high water tables

Much of Southwest Florida is low and flat. A 2018 study prepared for Lee County in part of North Fort Myers listed low elevation and a high water table among the reasons septic systems there did not protect local water quality, and UF/IFAS notes that septic drainfields need at least 24 inches of unsaturated soil to work properly. That affects both sewers and septic systems, because soil that is already wet has less room to take more. We mention it as general background, not as a measurement for any address. The soil and water table at your property may differ.

Rain can also move a clog

Sometimes the rain is not the cause at all. A partially blocked drain may have been getting worse for months, and a day of heavy use, with guests, laundry and a lot of showering because the weather kept everyone indoors, tipped it over. If the problem appears after a rainy week but has also been slow for a while, consider whether the rain was the trigger, not the cause. See clogged drain repair.

What to do when drains back up during a storm

• Reduce water use. Delay laundry, dishwasher loads and long showers.
• Do not use chemical drain cleaners. They do not help with a sewer or septic overload and they are hazardous.
• Keep people and pets away from any wastewater. EPA says rugs, curtains, flooring, wallboard and upholstered furniture that come into contact with sewage usually must be replaced.
• Photograph the problem and note the time. It helps the provider and, if relevant, your insurer.
• Find the cleanout. The FAQ explains where it may be.
• If you are on city sewer and many neighbors have the same problem, tell your utility.
• If you are on septic, call a provider who handles septic systems. A drain cleaner may not be the right choice. Do not have the tank pumped during flooded conditions without advice.
• If wastewater is coming into the house, read the sewer backup page.

When to look at the line afterwards

If a drain backs up after rain once and then behaves well, it may have been a one-time overload. If it happens every wet season, or it happens in light rain, it is worth finding out why. A camera inspection can show a cracked pipe, a root intrusion or a sag. Roots are covered on the root intrusion page.

Frequently asked questions

Why do my drains back up only when it rains?

Rainwater can enter a sewer system and overload it, or saturate the ground around a septic field so that it cannot take more water. A weak spot in your own line can also let groundwater in. A camera look can help tell which.

Is a rain backup my fault?

Not necessarily. A system overload is not caused by anything a homeowner did. A problem in your own lateral is the property owner's matter. A provider can help you work out which you have.

Should I pump my septic tank during a flood?

According to EPA, pumping a tank when the drain field is flooded or saturated is at best temporary, and it could damage the tank. Reduce water use and ask a septic professional for advice.

Can a drain cleaner fix a rain backup?

Only if the cause is a clog in your line. If the cause is an overloaded sewer or a saturated septic field, cleaning will not fix it.

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