If you've ever had water from the sewage system back up into your basement, you already know how much time and trouble backwater valves can save you. Municipal sewer lines can become blocked or overwhelmed. When this happens, the water backs up along the system into the houses that are connected to it, through floor drains, toilets, and bathtubs-which can be a nasty surprise.
This problem often occurs in low-lying areas where the plumbing in the basement sits at a lower level than the sewer line in the street. Many areas now mandate backwater valves in vulnerable areas, to stop the damage caused by several inches of sewer water in house basements. These valves have a flap inside them that lets water flow out unheeded, but snaps closed if water attempts to flow back the other way. They need to be installed in the house's main sewer line or a drain line, in a position to block sewage from entering the basement plumbing.
This is a complicated job, especially if the right location for the valve lies under a concrete basement floor. You should call in a plumber to set the right location for the valve and to install it. (If it's installed in the wrong place, it might stop sewage from coming out through a floor drain, but reroute it into a toilet or washing machine.)
Check with your county or municipality to find out what its policy is regarding backwater valves. In some areas, backwater valves are required by building codes, but in others, their use might be restricted or bound by building permits.
Backwater valves are also available to protect individual drain lines within the house, and they can be useful if you can't install a valve on the main line. You need to understand, though, that blocking the flow of sewer water into one area will force it out of another. You may, for example, choose to protect a floor drain in a utility room that contains your furnace, or to protect the drain pipe that leads out of your washing machine, but you'll have to realize that the wastewater will then likely appear in a downstairs bathtub, or back up through a toilet. It's a good idea to consult a plumber or house inspector before installing backwater valves anywhere so that you understand potential consequences.
A backwater valve installed on the main sewer line will be more effective than one installed at a floor drain-but check your local building regulations first.
Most basements will have a floor drain somewhere — usually in a furnace or utility room. The drain is there to handle floods from within the house, such as a broken hot-water heater or washing machine, so a backwater valve installed in the drain needs to operate properly, letting water out but not in.
Valves are available in a range of sizes, so measure the interior diameter of the drain pipe that you want to seal, and buy the right valve for the type of installation (in this case, a floor drain). If the drain is an odd size, you may need to buy a sleeve that fits into the pipe and forms the right size opening for the valve. Lift up the grate from the drain, and clean the top of the drain line. One common backwater valve uses a rubber gasket to seal the gap between the valve and the pipe. Drop the valve into place and tighten the screws on its top until the rubber gasket presses out to meet the pipe. Hold the valve in place and level while you're tightening the screws.
Check that the valve flapper is working freely before pouring some water through the valve to test that it's draining properly. You'll also need to check the valve periodically, to clean out any debris that might have fallen onto it.