Video on Facebook

Fredrick Maryland is experiencing severe flooding and one home owner has flood water gushing in the house through the toilet. You can view the video from the Facebook link above. What I don’t understand is how water can come backwards up the toilet? Aren’t there traps and other things that create suction or a seal? I’m not a plumber but I thought the pipes were designed to prevent something like this.

enter image description here

  • you can have a one-way valve installed in the sewer line
    – jsotola
    May 19, 2018 at 21:33

3 Answers 3


The trap on a toilet (or any plumbing fixture) prevents sewer gas from escaping. It's a very low-pressure thing, consisting solely of a small amount of standing water filling a low spot in the drain. Any force capable of moving a little water can overcome it, including air vacuum if it's not properly vented.

In the case of this flooding phenomenon, there must be a break in the city sewer line somewhere, and water standing above it. This obviously creates pressure that will escape at any point below its surface level.

  • Shouldn’t there be a cut off valve then for the drains too? May 19, 2018 at 17:48
  • There might not have to be a break, some wastewater and stormwater systems are combined, and/or have combined overflow systems. Frederick MD could have a CSO. May 19, 2018 at 21:25
  • 1
    Research shows that your guess of stormwater infiltration into the sewage system is more likely. A Maryland overflow database search shows that CSOs only seem to occur in Allegheny County (e.g., the City of Cumberland, out west). May 19, 2018 at 21:34

Most houses only have traps installed, which prevents sewer gasses from entering the building. Some municipalities do require backflow prevents, which prevent sewage from entering the building. They do have a couple of drawbacks, though. There are location and slope restrictions on where they'll work, which can make installation in existing construction difficult. They're also a mechanical device, which can fail. When that happens they do a bang up job of preventing sewage from leaving the building.

It's worth mentioning that combined storm and sanitary sewers used to be very popular, and are still fairly common. In some cities they're still working to disconnect downspouts from the sewer system. They are out of favor now for obvious reasons, but not every city is has separate systems for storm water yet.

  • The back flow devices do need to be cleaned or they may not stop water from coming or out just as Mathew stated. The need to be accessible for cleaning and having this happen once I would be installing one in the spring. No way I would try with all the "stuff" coming back up the line now.+
    – Ed Beal
    Nov 9, 2018 at 15:27

Happened in 2013 in our city. To many sewer and sanitation lines were mixed, broken etc. and 4-6 inches of rain in 90 minutes had 2 feet in most basements, all coming in through the plumbing.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.