>>2910429
>No showers were taken, no load of laundry, dishwasher during a heavy rain?
It depends a lot on what the source of the flooding is and what's going on downstream of the valve -- check valves can work with very low head differentials but keep in mind they are designed to prevent water from flowing the opposite direction. In other words, if there is a hydraulic head on the sewer side of the check valve that's higher than on the house side, it's going to want to stay closed. Now even without overhead sewers, your house is probably up higher than the top of the sewer main, so a check valve should be okay most of the time. However, keep in mind that you're already seeing cases of several inches of water inside your basement -- which means the HGL in the sewer was that much higher.
If it's sewer surcharge only due to a sudden and intense rain, then the surcharge may only last for a few minutes -- enough to flood you but not enough to cause a problem with a check valve. If it's a river flooding and backing up through the sewer system, then you could have a problem. Municipalities and counties in general are moving away from combined sewer systems because of issues like flooding as well as the cost of the additional sewerage that needs treating from collecting even a light rain.

For reference, here's a pamphlet from a municipality I'm familiar with; one of my coworkers used to live there and he had an issue whenever the Des Plaines River started getting near flood stage as it was combined sewer. He did the option on Figure 2 on Page 2.
https://www.desplainesil.gov/home/showpublisheddocument/564/637590123987800000

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My house is a small ranch built in 1950. I have been here for 13 years, the previous owner was here for 15 years, and she bought it from her aunt and…