I have a 30 +/- year old gate valve on my sewer outflow line. It is outside my house about 6 to 8 feet below ground. Even with the valve fully closed, the city sewer has backed up into my basement on two different times when their has been large amounts of inflow from heavy rains. Can these valves go bad when they are not used very often?

Kind of.. They tend to build up debris inside that prevents the flapper in them from closing fully. If you can disassemble it, you may be able to flush out the solids with a garden hose. It won't be easy or pleasant no matter what you do. I am assuming that this is the automatic type with a manual gate that can be operated?



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I am not sure of the "gate valve" that you are talking about. There are "back water valves that are basically a check valve that can be installed in sewer lines. If it were my home I would have it dug up and replaced. Sewage entering a home is a health risk.



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If it is in fact a gate valve, yes, they build up iron, rust, minerals inside and will not shut off totally over time. I would want a swing check valve if I understand your situation. Research Check Valves for sewer lines.