Why Drains Back Up When It Rains, and What You Can Check

Sewer pipes aren't built for rain

A sanitary sewer carries wastewater from toilets, sinks, tubs and washers to a treatment plant. Rain is supposed to go somewhere else: a storm drain, a ditch, a creek or your lawn.

Georgia's plumbing code is the International Plumbing Code with Georgia amendments, and Georgia has adopted the 2024 edition. The Georgia Department of Community Affairs (DCA) gives January 1, 2027 as that edition's effective date in its most recent announcement (December 2025), while an earlier DCA memo said January 1, 2026, so your city or county building department can tell you which edition it enforces (Georgia DCA). The 2024 code's storm drainage chapter is blunt: "Storm water shall not be drained into sewers intended for sewage only" (Section 1101.3). For one- and two-family homes, it allows storm water to discharge onto flat areas such as lawns or streets, as long as it flows away from the building, where the local authority approves (Section 1101.2) (ICC).

Rain gets in anyway. EPA lists leaky sewers that let in stormwater and groundwater, and connections from sump pumps, roof leaders (downspouts), foundation drains and area drains, among the problems that can overload a sanitary sewer and cause overflows (EPA). Every gallon of rain in the pipe takes up room meant for sewage.

Inflow and infiltration: how rain gets in

Engineers split the extra water into two kinds, and federal rules define both (40 CFR 35.2005):

• Infiltration is groundwater that seeps into the sewer system, including service connections and foundation drains, through defective pipes, leaky joints, bad connections or manholes. It rises when the ground is soaked.
• Inflow is water that pours in from sources such as roof leaders, cellar drains, yard and area drains, manhole covers, cross-connections from storm sewers, catch basins and surface runoff.

Both can start on private property. A cracked sewer lateral lets groundwater in, and a downspout piped into the lateral sends a whole roof's worth of rain straight into the sewer. Even a small opening counts: Clayton County Water Authority warns that broken cleanout caps can let stormwater in and lead to an overflow (CCWA).

Timing matters, too. Before Tropical Storm Fred in August 2021, DeKalb County warned that because the ground was already saturated, runoff from the storm could cause sewer overflows and spills, and its chief executive called stormwater intrusion a major source of the county's overflows and spills (DeKalb County).

Why the backup ends up in your house

When more water reaches a sewer main than it can carry, the main fills and the level rises in everything connected to it, including manholes and the service lines from houses. Sewage then escapes at the lowest opening it can reach. Sometimes that's a manhole in the street; if your basement floor drain or lower-level shower sits lower than the top of the next manhole upstream, it can be your house instead.

That elevation is why low fixtures get special attention: Clayton County Water Authority, for one, requires a backflow device on the lateral when a home's lowest fixture sits below the next upstream manhole, and the backwater valve guide explains how these devices work (CCWA).

Rain can also expose a problem in your own line. A lateral that is partly blocked by roots, or that sags and holds debris, may handle everyday flow but fail when groundwater seeping through cracks adds to it. One clue: if only your house backs up in storms and the neighbors are fine, the trouble is more likely in your own line than in the main. Clayton County Water Authority lists heavy rainfall overloading the system alongside blockages and tree roots as causes of backups (CCWA).

What Georgia rules say about downspouts and drains

Two layers of rules apply, and both vary in how they are enforced.

The plumbing code. As noted above, Georgia's code (2024 IPC, Section 1101.3) bars draining storm water into sewers meant only for sewage (ICC). Cities and counties can also have local amendments on file, which DCA has asked them to review against the 2024 codes (Georgia DCA), so requirements and enforcement can differ from place to place.

Your utility's sewer use rules. Each utility sets its own limits on what can go into its pipes. Augusta-Richmond County's utilities code, as posted on the city's website, prohibits putting storm water, surface water, groundwater, roof runoff or subsurface drainage into its sewer system unless the utilities department specifically authorizes it (Title 5, Section 5-3-4) (Augusta). DeKalb County has asked homeowners to help reduce spills by eliminating illegal connections to the sanitary sewer and replacing missing and broken cleanout caps (DeKalb County).

Other Georgia utilities have their own versions, with their own inspection programs and penalties. To check yours, look on its website for a sewer use ordinance or sewer regulations, or ask customer service.

A checklist for your property

Most of this takes one walk around the house on a dry day.

1. Follow every downspout. If it ends at a splash block or an extension that spills onto the lawn and away from the house, that's the kind of discharge the plumbing code allows for one- and two-family homes. If it disappears into a pipe in the ground, find out where that pipe goes: a storm drain, an outlet elsewhere in the yard, or, the problem case, your sewer line. A plumber can trace it with a camera or a dye test, so don't assume.
2. Check yard, driveway and stairwell drains. Area drains are on EPA's list of connections that can overload a sanitary sewer (EPA). Ask where yours discharge.
3. Find where the sump pump discharges, if you have one. EPA lists sump pump connections among the same problems (EPA), so the pump should send groundwater outside and away from the house, not into a laundry sink, floor drain or sewer pipe.
4. Look at every cleanout cap. Each should be present, intact and closed. Replace a cracked or missing cap, which DeKalb County and Clayton County Water Authority both flag as a way for rainwater to get in, and keep caps on during storms. The cleanout guide shows what to look for.
5. Note your lowest drains. Basement floor drains, showers and toilets on the lowest level are the ones at risk when the main fills up.
6. Write down the pattern. Does it happen only in heavy rain? Only at your house, or along the whole street? Does it start hours after the rain begins? That record helps a plumber or the utility find the cause.
7. If it's only your house, have the line inspected. A camera inspection can show cracks, roots and offset joints that let groundwater into your lateral.

What to report, and to whom

• Sewage coming out of a manhole or cleanout, or running into a storm drain, ditch or creek: that's a sewer overflow, so report it to your sewer utility right away. Atlanta's Department of Watershed Management lists sewer overflows among the emergencies to report to it immediately (Atlanta DWM), and DeKalb County has asked the public to report any sign of overflows and spills to its 24-hour line (DeKalb County). Your utility's emergency number is on its website and bill.
• Sewage backing up inside your house: stop using water and follow the sewer backup guide, then let the utility know, especially if neighbors are affected.
• Backups that return with every big storm: report each one so the utility has a record of where trouble repeats. DeKalb, for example, also uses flow monitors to find where stormwater is getting into its pipes (DeKalb County).

Whether a repair is your job or the utility's depends on where the problem is and on your utility's rules; see who is responsible for the sewer line in Georgia. Clayton County Water Authority notes that typical homeowners insurance does not always cover sewer backups (CCWA), and the sewer backup insurance guide explains your options. This is general information, not legal or insurance advice; your utility, a licensed plumber or your insurer can advise on your situation.

Bottom line

Rain-driven backups happen because stormwater gets into pipes built for sewage, through cracks, connections and open cleanouts, and the overflow comes out at the lowest openings it can find. Check your own property for rainwater connections and open cleanouts, know how low your drains sit, and report overflows to your utility every time.