Most sewer backups start at your house and work outward. Occasionally it goes the other way, and that is the one nobody plans for.
What happens when the city main plugs
The municipal main carries everything from every house on the street. If it blocks, that flow has to go somewhere, and it goes to the lowest opening it can find.
That is usually not the house that caused it. It is whichever home on the run sits lowest, through the floor drain in the basement. Your line was fine. Your fixtures were fine. You were simply at the bottom of the hill when something upstream gave way.
It is not common. It is also not theoretical, and in a city built on a slope the geography decides who is exposed rather than the condition of anyone’s plumbing.
What a backflow preventer does
It is a valve on your sewer lateral that allows flow out and blocks flow coming back in. Normal use, it sits open and you forget it exists. Reverse pressure, it closes.
It does nothing for a blockage in your own line. That is worth being clear about, because the two problems look identical from the basement floor. A preventer is specifically for the case where the trouble is on the municipal side.
Duluth sometimes requires one
Where a home’s sewer sits at a similar height to the city main, and the area has a history of the main plugging, we have seen the city require a backflow preventer as a condition of the work.
Two conditions together, not either alone. Plenty of homes sit low without ever being asked, and plenty of plug-prone areas have houses high enough that it never comes up.
If this applies to your address, you will find out when the permit goes in rather than having to work it out yourself. Worth knowing in advance that it exists, because it is not a line item most people have budgeted for.
How to tell whether you are the low house
You do not need a survey. Three things give it away.
Your basement floor drain is the lowest fixture in the house and sits near or below the level of the street at the front.
You are at the bottom of a run. On a hill, look at where the water would go. If your house is the last one before the ground flattens, you are a candidate.
It has happened before. Either to you or to a neighbour. These events repeat, because the conditions causing them rarely change on their own.
What it does not replace
A preventer protects against one specific failure, and it is the rarer of the two. The common cause of a basement backup is still your own lateral: roots at the joints, a collapse, or a house trap catching everything that passes it.
Fitting a valve and leaving a failing lateral in the ground solves the problem you were unlikely to have and leaves the one you were.
Which is the normal order of operations. Find out what condition your own line is in first, then decide whether the municipal side is a risk worth a valve. The camera inspection that answers the first question is free, including the locate and the written report, and it tells you where you sit relative to the main at the same time.
If it has already happened to you
Two things matter, in this order.
Get the cleanup done properly. Sewage in a basement is a contamination problem before it is a plumbing problem, and it needs handling as one.
Then find out whether it was your line or the main. The answer changes everything about what you do next, and you cannot tell from the mess. If the line was yours, you are looking at repair options. If it was the main, the conversation is about protecting yourself from the next one.