Been trying to figure this one out. My toilets "boiled"/burped when the City flushed the main sewer line at the street. We saw later that it pushed solid waste into the toilet housing that sits on the floor, and shot liquified waste between the flooring and the subfloor halfway across the room. The toilet bowl water remained clear. The waste was discovered when the toilet and then the flooring got removed to investigate.

The event happened two weeks after new flooring and a new toilet went in. A remediation company cleaned up.

My Qs: Why is my house the only one this happened to on the block? What issues or mechanics would have allowed the waste to enter my house?

Thanks for any professional ideas you have about this. All the words I've used, like "back flow," "boiling/burping," I've learned since this happened. Just trying to piece it together, find potential causes, and prevent it happening again.

The best way to prevent other people’s sewage from entering your sewer line is to install a back water valve

As for why it only happened to your house… maybe it didn’t or luck of the draw/ you may have been the closest house to the clog at the street

A friend in Durango Colorado watched a city truck insert a pressurized sewer line cleaner down the block, then a moment later experienced toilet volcanoes in his home. Some neighbors experienced the same thing, others did not.

When the new flooring was installed did they raise the toilet flange to set on top of the new flooring? If not, that may have created a weak seal that allowed the leak to occur in that area.

Helpful to know this is a possibility. They installed the flooring, then installed a new toilet on top of it.

About the flange, there are two toilets in the house, and only one had the sewage blow. (The remediation people tested for moisture in the 2nd bathroom and found none.) Considering the force of the city's cleanout, wouldn't the second toilet have blown sewage also?

Thanks. Gives me some info about the street.



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That’s frustrating — and honestly pretty rare — sewer event. The fact that it happened only to your house gives some strong clues. Here’s a breakdown of what most likely happened and why your house was uniquely vulnerable.

What Probably Happened:

When the City flushed the main sewer line (using high-pressure water jets or a vacuum truck), it pressurized the public main sewer, and that pressure pushed back into your private lateral — the 80 ft clay line from your house to the street.

If your lateral or plumbing system lacked proper venting or backflow protection, that pressure had nowhere else to go but back up through your toilet, which is often the lowest fixture in the system.

Why Your House?

Here are the most likely contributing factors:

1. No Backwater Valve (BWV)

This is the #1 reason.

Backwater valves are check valves that allow flow out but prevent flow in from the public sewer.
Most older homes (like your 1969 build) don’t have one unless it was added later.
Without it, pressure from a main flush can push sewage into the house.

If you don’t have one, adding one can prevent this from ever happening again.

2. Shallow or Direct Drain Line Slope

If your sewer line is relatively flat, short, or close to the main, pressure can push material backwards more easily.
Your lateral may have a belly or low spot that allows material to sit and get pressurized.
Clay pipes with joint gaps may also allow more air pressure transfer.
3. Toilet Seal Compromised or Poorly Installed

Since this happened two weeks after new toilet + flooring:

The toilet wax ring or seal may have failed, or wasn’t seated right.
That allowed pressurized, liquified waste to escape under the toilet base, rather than only up into the bowl.
If bowl water stayed clean but waste got under it — this is a sign the seal failed under pressure.
4. Ventilation/Stacking Issues

If your plumbing vent system is undersized, blocked, or altered during reno, pressure can't dissipate properly.
That could force air or water back into fixtures.

How to Prevent It

Install a Backwater Valve
In-line valve in your main sewer lateral.
~requires yard/basement access.
Check Toilet Installation
Re-seat with new wax or rubber seal.
Consider a waxless seal or extra-thick ring if floor height changed.
Inspect Sewer Line (Camera)
Look for belly, low spots, blockages, or unusual slope.
While you’re at it, confirm if a BWV already exists.
Ask City if Event Was Logged
They may keep records of maintenance that led to this issue — and might accept some liability (worth asking if damage was extensive).

You likely don’t have a backwater valve, and your new toilet’s seal may have failed under pressure from the City’s mainline flush. This allowed liquified waste to exit under the toilet and into your flooring. Your neighbors may have BWVs, better slope, or higher drain elevation — which kept them safe.

Hope this helps.

Have a blessed day!



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