If sewage is backing up into the house, stop using water immediately — every flush or drain adds more volume the system cannot currently handle — and call a septic professional. Do not enter an area with standing sewage water without protective equipment, since it is a health hazard, not just a mess.
Backups almost always start at the lowest drains in the house (a basement floor drain, a downstairs bathroom) because those are the first point where backed-up wastewater has nowhere else to go. That is also a useful diagnostic: if only the lowest fixtures are affected while upper floors still drain normally, the blockage or overload is downstream, likely at the tank or beyond.
A backup is most often caused by a full tank that has not been pumped, a failing drainfield that can no longer accept flow, or a physical blockage in the line between house and tank. A professional can determine which quickly, which matters because the fix (a simple pump-out versus a drainfield repair) is very different depending on the cause.
Once resolved, any area that had contact with backed-up sewage should be properly cleaned and disinfected, since raw wastewater carries pathogens that ordinary cleaning products are not designed to handle.
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Stop using water in the house right away, since every additional flush or drain adds volume the system can't currently handle, and call a septic professional. Avoid contact with any standing sewage water.
Most often a full tank that hasn't been pumped, a failing drainfield that can no longer accept flow, or a physical blockage between the house and the tank; a professional can determine which quickly.
Guide last reviewed 2026-08-31.