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Basement Flooding:  Sewer Backup - A Guide to Floodproofing Your Basement 

Often basement flooding is caused by two sewer systems being interconnected. Some houses have downspouts, footing drain, and/or the sump pump connected to the sanitary sewer service. During a heavy rain, storm water enters the sanitary sewers, causing backups into one house and overloading the main lines, contributing to backups in other houses.

Sewer backups can also be caused by events not related to storms or flooding. Individual service lines can be plugged by grease, waste, tree roots, breaks in the pipe, or saturated ground. Proper maintenance, like pouring tree root killer down the toilet each year, can prevent most of these problems. The sewer mains can also become plugged by the same causes as well as vandalism or illegal placement of items in manholes. These problems can be fixed by the owner or the City, depending on where the stoppage occurs.

This guide focuses on protection measures that deal with sanitary sewer backup that occurs when the sewer main is overloaded and backs up through the sanitary service line into the house.

There are four ways to stop sewer backup: floor drain plug, floor drain standpipe, overhead sewer, and backup valve. Each of these measures work for buildings with basements or below-grade floors.

Floor Drain Plug

The simplest way to stop sewer backup is to plug the opening where it first occurs. This is at the floor drain, the sanitary sewer system’s lowest opening in the house. Commercial plugs are available and can be placed in the floor drain below the grate. Bolts on metal end pieces are tightened causing a rubber gasket to expand and seal the plug in the pipe.

A plug stops water from flowing in either direction. Therefore, if the laundry tub overflows or other spillage occurs, it will stay in the basement unless the plug is removed. Because of this, it may be best to leave the plug out under normal circumstances and put it in place only during heavy rains.

The advantage of the plug is its low cost and ease of installation. A standard floor drain plug can be purchased at most hardware stores for $5-$10.

One variation is a plug with a float. It allows water to drain out of the basement. When the sewer backs up, the float rises and plugs the drain. A float plug permanently installed will not interfere with the drain’s normal operation.


A plug left in the flood drain may contribute to a wet basement if spillage cannot drain out. Float plugs are known to have been jammed open by a small amount of debris.

A floor drain does not stop backup from coming out of the next lower opening, like a laundry tub or basement toilet. Sealing the base of the toilet to the floor will protect you until the water backs up higher than the top of the bowl.

A plug does not tell you if there is a problem in your sewer line. If the plug is not tight enough, pressure can eject it. In older houses, the sewer lines under the basement floor may be clay tile. A buildup of pressure can break them. In new houses, they are cast iron under the floor and less likely to break.


A standpipe is an inexpensive alternative to a floor drain plug. A "donut" with metal end pieces and a rubber gasket in the middle is placed in the floor drain. A length of pipe is placed in the "donut hole." The "donut" can be purchased for about $10. A three-foot length of pipe costs less than $5.

When the sewer backs up, the water stays in the pipe. Water pressure can build up to blow a standpipe (if properly installed) out of the floor drain. The system works unless the backup is so deep that it goes over the top of the pipe.

One advantage of the standpipe over the floor drain plug is that the overflow acts as a safety valve. Flooding in the basement equalizes water pressure on the walls of the floor, minimizing the chance of a cracked floor from broken pipes underneath.


A standpipe left in the floor drain may contribute to a wet basement if spillage cannot drain out. A standpipe only protects up to its height, normally three feet. Deeper flooding will flow over the top. (A taller standpipe is not recommended because it can result in too much water pressure on your pipes).

Overhead Sewer

An overhead sewer acts like a standpipe but without the problems. A sump is installed under the basement floor to intercept sewage flowing from the basement fixtures and the basement floor drain. An ejector pump in the sump pushes sewage up above the flood level. From there it can drain by gravity into the sewer service line. Plumbing fixtures on the first floor continue to drain by gravity to the service line.

It is unlikely that the sewers will back up above the ground level. If water does go higher, a check valve in the pipe from the ejector pump keeps it in the pipes. Backed up sewage is enclosed in the sewer pipes so there is no worry about overflowing laundry tubs or basement toilets.


The ejector pump requires maintenance and electricity to work properly. The basement is disrupted during construction. The contactor may have to run the overhead pipes through one of more basement rooms, although often they can be camouflaged. This work requires a licensed plumber and a building permit.

During a power outage, the ejector pump won't work. But this only limits the use of the facilities in the basement that need the pump. The upstairs plumbing still works and the sewer is still prevented from backing up.

Although more dependable than a standpipe, an overhead sewer is typically more expensive. A plumbing contractor must reconstruct the pipes in the basement and install the ejector pump. It typically costs $2,000-$5,000.

Backup Valve

A backup valve stops the water in the sewer pipes. Older versions of the approach were located in the basement and relied on gravity to close the valve. If debris got caught in the flapper, the valve did not close tight. Because of its unreliability, valves were discouraged and even prohibited in some communities. A newer "balanced valve" has corrected these design shortcomings. A system of counterweights keeps it open all the time so debris won’t catch and clog it. When the sewer backs up, instead of relying on gravity, floats force the valve closed. It is usually installed in a manhole in the yard so there is less disruption during construction and no concerns over breaking the pipes under the basement floor.

As with overhead sewers, a valve is fully automatic. It can even work when there is surface flooding. The owner can still use the sanitary sewers during flooding as long as there is power to run the ejector pump, which forces wastewater into the sewer line when the valve is closed.


The ejector pump and valve require maintenance. This work requires a licensed plumber and a building permit.

The cost of this type of backup valve is comparable to the cost of an overhead sewer, in the $3,000 to $5,000 range.


This guide is designed to give the reader an overview of things that can be done to protect a property from damage from the type of surface water flooding and sewer backup that faces many residents in the City of Wood River. The information provided is based on careful research and input from experienced professionals. The reader must assume responsibility for adapting this information to fit his/her conditions. This guide is not intended to replace the advice and guidance of an experienced professional who is able to examine the building and assess the needs of the particular situation.

For more information, contact the Department of Public Services at (618) 251-3122 or the Department of Building & Zoning at (618) 251-3100.