How Does a Basement Floor Drain Work?

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Basement floor drains don't get a lot of attention until the sewer backs up or the basement smells like an outhouse. Many basement floor drains tie directly to the home's sewer system, but in some communities, local building codes require floor drains to run to a sump pit, where a pump lifts the water to the exterior surface of the house.

Anatomy of a Floor Drain

Situated at the lowest point on the basement floor, the floor drain serves as an outlet for water from a leaking water heater, condensation from an HVAC unit or even seepage due to heavy rain. If the drain connects to the sewer system, it's suitable for draining a washing machine, water softener or basement laundry sink. If the floor drain connects only to a sump pit, it's probably against local ordinances to drain any water that contains detergents, salt or chemicals.


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If the basement floor is lower than the spot where the main sewer line exits the home, the floor drain could connect to a third type of drainage system – a sewer pit with an ejector pump. If the drain runs to a sewer pit, which is not the same as a sump pit, it's permissible to drain a washing machine or sink in the floor drain.

Sewer-Connected Floor Drains

From the basement, only the cover grate is visible. A catch bowl, drainpipe and plumbing trap lie beneath. If your floor drain has an additional cleanout, you'll see two outlets when you remove the grate; a large drain hole at the bottom and a smaller one, with a plug, on the side. The catch bowl and drainpipe are typically made from one of three types of material.


  • PVC: most newer drains
  • Cast iron: older drains
  • Clay tile: older and less common

Floor Drains and Sewer Gas

If your floor drain connects to the sewer system and you smell sewer gasses, the trap beneath the drain is dry or the plug for the cleanout is missing. If nothing drains into the floor drain, prevent sewer odor by pouring a pitcher of water down the drain every couple of weeks.


If the cleanout plug is missing, replace it with a rubber expansion plug from a plumbing supply store.

Clogs and Backflow

Over time, debris and soap residue can accumulate in a drain line, slowing down or blocking drainage. Periodic use of liquid chemical clog removers can prevent many clogs, but you might need a powered plumbing snake to chop away stubborn blockages further down the line.


For floor drains that tie into the sewer, backflow is always a concern. If a clog occurs in the home's main drain line and sewage has nowhere to go, it can seep up through the floor drain. You can install a backwater valve, also called a backflow valve or check valve, in the drainpipe. When sewage backs up, a float in the valve rises and seals the drain.

Backflow is less worrisome in floor drains that connect to sump pits, but in times of heavy rain, water from the foundation's drain tile, which often discharges into the same pit, can back up if the sump pump fails, so it's not a bad idea to install a backwater valve there as well.



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