My basement flooded recently and I'm trying to determine why.

The basement has poured foundation walls and a slab. The slab is non-structural, and was poured after the foundation walls. There is a tile drainage and sump pump system. It's a new build.

We got some heavy rain that resulted in the basement flooding. Some neighbors who also had flooding, suspect a sewer backup.

The sump pump continued to work during the flooding, but perhaps was overwhelmed. I still see water seeping between the foundation wall and slab, and the slab is discolored where water appears to be seeping up. The water was clear and didn't smell. If the water came from the sewer, I would imagine it would have had to have come from the HVAC condensation drainage.

How can I tell whether the flooding was from a sewer backup or from a failure in the tile drainage/sump system? If there's no way to determine why it flooded before, is there anything I can do to find out if/when it happens next time?

  • Just because it didn't stink, doesn't mean it's not sewage. If your locality has combined storm and sanitary sewers, then an abundance of storm water may cause an overflow of mostly storm but some waste water. Is the HVAC drain really connected to the sewer? That seems odd to me as usually you don't want that.
    – jwh20
    Commented Mar 22, 2020 at 19:15
  • You mentioned heavy rain. Did it flood in your area?
    – Ack
    Commented Mar 22, 2020 at 19:19
  • Is there a floor drain ? That is the typical source if it is sewage. Commented Mar 22, 2020 at 21:01
  • @Ack - My neighbor's yard had standing water, but generally, no, there was not flooding.
    – DIYoutube
    Commented Mar 23, 2020 at 16:33
  • 1
    In most localities they do not permit "clean" water such as HVAC condensate to enter the wastewater stream. It should go to a storm drain or other outlet such as a drainage tile or ditch on your property. But also if the sewer backs up, you get what you have.
    – jwh20
    Commented Mar 23, 2020 at 16:46

1 Answer 1


If it was a sewer back up you can install a check valve on your waste line, these need to be accessible as they require maintenance but will prevent the sewer system from backing up. Floor drains & showers are the first to flood in a back up but then toilets and washing machines, sinks all become possibilities.

Depending on the type of condensate line moving the trap higher may be the only safe method as check valves on condensate lines tend to fail. Other than monitoring and identifying the water entrance point it may be hard to figure out, if the sewer was backed up and your sump pumps into it, where would the water go? Could the pump itself not only have been overwhelmed but provided a path for water ingress (if their is not a check valve in the pump). I have seen sewers back up and put several feet of water in a basement with the water appearing to be clean.

  • The sump pumps to the street and is separate from the sanitation system. There is a check valve between the sump and the discharge line that goes to the street. So it could be that the sewer backed up, the sump was overwhelmed, or both happened independently.
    – DIYoutube
    Commented Mar 23, 2020 at 15:24

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