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If I have a sewage backup from the city sewage line during a storm surge, and I cover up my basement floor drains and sink, will the sewage just keep going up the line into my upper floors and flood upstairs?

I'm looking to add a check valve to the sewage main, but since that will require a lot of concrete to be dug, seeing if the short-term solution to add check plugs to the floor drains in the basement are a viable alternative.

If there is any risk of it going upstairs, I will not take the chance, but not sure if I understand the physics of how the liquid flows. On one hand it might not go up since it's higher than the other buildings, but on the other than maybe the pressure will just make it flow up anyhow.

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  • If the pressure pushing the water/sewage is high enough, it will keep going up till the weight of the water in the pipe/s overcomes the pressure. Most sewage backups should be of a low pressure, but I would not chance it.
    – crip659
    Commented Sep 7, 2021 at 14:00
  • Has this happened? There are storm sewers and sanitary sewers...two completely separate systems. I don't think a storm will cause a sanitary sewer to backup.
    – Steve Wellens
    Commented Sep 7, 2021 at 14:00
  • @SteveWellens Most places this is true, but even water from storm sewers probably not that nice. Is it still sewage if it comes from a storm sewer?
    – crip659
    Commented Sep 7, 2021 at 14:04
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    In some localities, a severe storm can lead to an overflow from storm sewer system to sanitary sewer system
    – Phil Freedenberg
    Commented Sep 7, 2021 at 14:26
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    Storm vs. Sewer. Best practice of course to keep the 2 separate. But that wasn't the practice years ago when much of this infrastructure was installed. It took the town I live in years and years to separate the 2....and you can just imagine the expense. So it wouldn't surprise me at all that there are many places that still have them combined.
    – George Anderson
    Commented Sep 7, 2021 at 15:52

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If you have a dedicated sewer line (in a remote area) and the surge/pressure is great enough, yes, the sewage will rise to the next level if your basement drains are plugged. If you live in a regular neighborhood, many homes share the main sewer line so if you were the only one blocking your drains in the basement, then you'd be OK and all the neighbors basements would be flooded. If they all blocked their basement drains, then the sewage could possibly rise to the next level but miles of pipes would have to be full.

"If there's any risk of it going upstairs" There's always a risk of anything happening. Areas have reported floods that haven't happened in 100 years, South Florida just had a condo collapse, so anything can happen. Check the archives for your area flooding, storm surges, etc.

If you do add the check/gate valve, make sure it remains accessible for maintenance.

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  • Where I used to live, all the sink drains had outdoor air gaps which would probably overflow first. (since there are no contamination issues here, the outflow pipe was "inside" the air gap and any backup would rise above its level, unlike in a faucet air gap) Is that not standard worldwide?
    – Criticizing Israel not allowed
    Commented Sep 8, 2021 at 9:23

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