Preventing Backflow

Patricia Skinner, Bankston, Jr., Joseph D., Reichel, Claudette Hanks  |  2/15/2005 10:06:58 PM

Download Publication ID: 2770

In most areas of Louisiana, the sewer system and rain drainage system are separate. It is not unusual, however, for floodwater to infiltrate the sewer system, causing it to back up into homes. These back-ups can occur both inside and outside the identified flood hazard areas and can be caused by things other than floodwater entering the system.

When there is more water outside than inside a floodwall, levee or building, water continually tries to get inside. Obvious paths of intrusion are sewer drains for the bathtub and toilet and drainage tubes in floodwalls and levees. Floor drains in some areas of buildings also could provide such a path. Any drain with its inside opening below flood level must be blocked. The drains may not be obvious – such as air-conditioning condensate drains – so look carefully.

Using Valves

A single valve installed in the main drain line can prevent backflow through several interior drains.

There are advantages and disadvantages to each of the three valve styles:

Flap valves are the least expensive but the most prone to failure. A bit of trash stuck on the valve gasket can prevent a flap valve from closing completely. The advantage of a flap valve, aside from its lower cost, is that it reduces back-flow without your intervention.

Gate valves and ball valves normally must be closed by mechanical action but can be automated.

Ball valves close by turning the valve handle a quarter turn; this rotates a ball within a drain tube to block flow. A 4-inch PVC ball valve coasts about twice as much as a flap valve. This valve provides a more positive closure than a flap and it will seal even in the presence of some debris.

Gate valves close by turning the handle several complete rotations to slide the gate across the drain tube. Gates provide the tightest closure possible with valves; the metal valve can overcome the resistance of small debris. A manual 4-in brass gate valve is three times the cost of a PVC flap valve. Hydraulically activated gate valves cost over $1,000. Installing any of these valves in an existing sewer line is equally difficult, since it requires digging up the sewer line. However, the benefits far outweigh the trouble and cost of preventing unhealthy sewage back-flow, even if you don’t keep surface flood water out of the building.

It may be beneficial to combine a flap valve (for automatic closure) with a ball or gate valve that requires manual closure but provides a more positive seal.

Using Caps and Plugs for Open-ended Drain Lines

When the outlet of a drain is exposed, as it is with drain tubes through a floodwall or levee, one simple solution to back-flow is to block the line with a threaded or unthreaded cap.

In a closed sewer system, a valve in the outside sewer line should prevent back-flow through tubs, toilets and other plumbed drains inside the building.

If you don’t have a valve, or the valve fails, you may be able to block the drain openings inside the building.

To do that, you’ll need to access the openings. For a tub or floor drain, that may mean removing the strainer. For a toilet, it means removing the toilet. (Don’t forget to turn off the water to the toilet first and disconnect the water inlet to the tank.)

A product sold for plugging drain pipe is a gripper plug. Tightening the screw causes the plug to expand and “grip” the inner wall of the pipe.

Seepage Barriers

The solution to seepage problems, if you have seepage, is to block underground flow with a barrier that increases the distance water must travel through the ground to get past your barrier.

Most Louisiana soils have more than 30 percent clay content, so seepage is not a significant problem for floods shorter than two days.

However, if the soil was imported for construction of the building, the rate of seepage should be determined. This is done using the percolation test commonly used to determine whether a septic system will drain properly.


  • Prevent sewage back-flow with a valve installed in the main sewer line.
  • For back-flow protection without human intervention, use a flap valve. Back it up with a positively sealing ball valve, gate valve or plug. More elaborate valve systems are available.
  • Caps or plugs may be used instead of valves on exposed storm drains to prevent back-flow. Flow is blocked in both directions.
  • On open-ended drain tubes, keep valve gaskets, cap seats and threads clean so they will function properly. Know where removable caps are kept.
  • In an emergency, prevent sewer back-flow by stuffing a plastic bag full of rags into the sewer pipe at the clean-out.
  • Never use pumps and barriers to create a water-level difference of more than 3 feet without proper design by a competent professional.

See PDF for illustrations.

2012 Insurance Reforms Impact Investment Decisions

Before you decide to invest in restoring or improving your home, ask your insurance agent about proposed increases in flood insurance premiums and how these can affect you. Subsidized rates are being phased out beginning in 2013, resulting in much higher premiums for properties that are too low in a flood hazard area.

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