Guide to Flood Proofing 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 or ABS plastic under the floor and less likely to break.


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