When the outlet of a drain is exposed, one simple solution to back-flow is to block the line with a threaded or unthreaded cap. This is often the case with drain tubes that pass through a floodwall or levee. Sometimes, though, you need to block a pipe that doesn't have enough pipe exposed to allow you to cap it. This is the case for drainpipes under the tub and toilet and possibly elsewhere.
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. These open lines are called points of entry for floodwater.
You’ll need to access the opening inside the house. 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.
The devices used to block water at points of entry go by different names: gripper plugs and test plugs are two. "Gripper" describes the action of the plug, as it grips the wall of the pipe it's plugging. "Test" refers to the fact that these plugs are used during construction to plug the plumbing lines so they can be tested for leaks under pressure.
The picture shows two styles of gripper plugs. Gripper plugs come in different styles and may be plastic or metal (with a rubber seal). Gripper or test plugs are installed by placing them in the drain tube and turning a nut. As the nut turns, it causes the plug to expand and "grip" the wall of the pipe.
Gripper plugs come in different sizes; be sure to get plugs that are the right size for all your openings. Some sizes are available at building supply stores; for others, you may need to visit a plumbing supply store. A 4-inch plug sells for about $4.
To block back-flow at the toilet using a gripper plug, remove the toilet and place the gripper plug into the exposed sewer pipe opening. Do not try to plug the toilet bowl.
In a pinch you can use a bag-o-rags to act like a gripper plug.
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