Backwater valve installation in Saint Paul, MN

Inside the valve body is a hinged flap, sometimes with a float. In normal use, water from your house pushes the flap open and flows out to the street. If the city sewer fills up and water starts flowing back toward your house, the flap swings shut and seals against the backflow. When the sewer drains down, the flap opens again on its own.

That simple action can prevent a basement full of sewage. The valve does not stop water that comes from inside the house, though. While it's closed, anything you drain has nowhere to go and can back up at the lowest fixture. During a big storm, it's wise to hold off on laundry, long showers and the dishwasher until the sewer has had time to drain down.

Most residential valves are made of PVC or ABS with a clear or removable top so you can see and clean the flap. Watts, among others, makes common residential models.

A backwater valve has to protect the drains that are below the level where sewer water could rise. Usually that means installing it on the main building drain, after the lowest fixtures connect, inside the basement floor. We break the concrete over the pipe, cut out a section, set the valve level and in the right direction of flow, and add an access box so the top stays reachable.

Placement takes some care. If upstairs fixtures drain through the valve, a storm closure can back them up into the basement. On some homes we arrange it so only the low basement drains run through the valve, while upper floors bypass it. We decide that after the camera shows how your branches join the main.

Not every home needs one, and a valve that isn't maintained can cause its own problems. They make the most sense where the risk is real and the stakes are high. A few questions help decide.

If the answers lean yes, we'll check the main line with a camera first. A valve needs a sound pipe and a reasonable slope to work. If the main has broken sections or heavy roots, it makes more sense to fix that at the same time.

• Has sewage ever come up through a floor drain, toilet or shower in the basement?
• Is the basement finished, or do you store things you can't afford to lose?
• Are there plumbing fixtures below the level of the street?
• Have neighbors had storm backups?
• Is your main sewer line due for repair or replacement anyway?

A backwater valve is a moving part in a sewer line, so it needs a look now and then. Paper, grease or a small object can lodge in the flap and hold it open, which defeats the purpose. Or debris can keep it from opening fully, slowing your drains. We suggest opening the access cover once or twice a year, especially before spring thaw.

Checking it is simple. Remove the clear cover or cap, look at the flap, and make sure it swings easily and seats cleanly. If it's dirty, rinse it with a hose. We show you how when we install it. If you'd rather not handle it, we can check it during any other visit.