First: stop adding to it
A blocked main means everything draining from the house is backing up behind a restriction. Every additional gallon raises the level and pushes it toward the lowest opening — usually a ground floor shower, tub or toilet.
Tell everyone in the house, because this only works if nobody runs a tap in the next ten minutes.
Stop the appliances specifically. A washing machine discharges a large volume quickly, and a dishwasher does the same on a cycle nobody is watching. These are the most common reason a manageable backup becomes sewage on a floor while everyone is in another room.
Do not keep flushing to “clear it”. That adds volume to a system that cannot take it.
If the level is already rising in a fixture, this alone often stops it getting worse.
Confirm it is actually the main
The response is completely different for a single fixture, so establish which you have.
One slow drain, everything else fine — that fixture's trap or branch. A plunger and a trap clean are appropriate. See clearing a drain properly.
Two or more affected at once, a toilet that makes a nearby tub gurgle, or water rising in a shower when the washing machine drains — a shared restriction downstream of all of them. That is the main.
The clearest test, if it is safe: run water in a ground floor tub or shower briefly and watch. If the level rises rather than draining, the main line cannot take it.
Symptoms show up in the lowest and closest fixtures to the main first, which is why a downstairs bathroom misbehaving while upstairs is fine is a textbook pattern rather than a coincidence.
Worth knowing
Do not plunge a main line blockage. The pressure has to go somewhere, and with the line full, that somewhere is up into whichever fixture sits lowest. People plunge a toilet and produce a backup in a downstairs shower.
Find your cleanout
This is the single most useful thing you can do while waiting, and most homeowners do not know where theirs is.
It is a capped pipe, usually 3 or 4 inches across, either just outside the house near a bathroom wall or out toward the property line. Sometimes there is one inside, in a garage or utility area. The cap may be plastic, brass or cast iron, and it is frequently buried under mulch, hidden by a shrub, or covered by a decorative rock someone put there years ago.
Clearing a line through a cleanout is straightforward. Clearing one without a cleanout means going through a roof vent or pulling a toilet, both of which take longer and cost more.
One warning: if the line is backed up, there is pressurised sewage behind that cap. Do not remove it yourself unless you are prepared for what comes out and are willing to be standing in it. Locating it is genuinely helpful; opening it is not.
Once this is over, mark it. A paving slab, a marker, or just a photograph and a note. Everyone who has hunted for a cleanout in the dark agrees it is worth five minutes on a dry day.
The three things that make it worse
Chemical drain cleaner. It cannot clear a root mass, a sag or a wipe accumulation, which are what actually block main lines. It sits in the standing water instead — still caustic — and becomes a hazard for whoever opens the line. Tell your plumber if any has gone down, because it changes how they approach the job.
Continuing to use water. Covered above, and it is the most common mistake.
Repeated clearing without diagnosis. A properly cleared line stays clear. If yours has backed up more than once, clearing it again is buying the same afternoon over and over. At that point a camera costs less than the next two visits, because it establishes whether you have a blockage or a failing pipe — very different jobs.
If sewage has come into the house
Treat it as a health matter rather than a plumbing one until the cleanup is handled.
Keep people and pets out of the affected area. Open windows and get air moving. Wear gloves and boots if you have to go in, and wash thoroughly afterwards.
Porous items that have been soaked — bath mats, towels, rugs, cardboard — are generally not worth saving. Hard surfaces can be cleaned and disinfected.
Photograph everything before you start cleaning. Insurance claims for sewage backup often depend on documentation, and many policies treat it differently from other water damage — some require a specific endorsement. Check yours before assuming you are covered.
Water that has gone under flooring or into a wall cavity needs proper drying rather than mopping. That is where the lasting damage comes from.
Afterwards, ask why
A main line does not block for no reason, and the answer determines whether this happens again.
Accumulation from grease and wipes — preventable by habit, covered in what never goes down a drain.
Roots, which enter through a crack or separated joint rather than breaking sound pipe. Cutting them treats this season only; sealing the joint is what ends it.
A sag or offset caused by our clay soil moving through wet and dry cycles. That produces repeat blockages in pipe that is otherwise intact, and no amount of care prevents it.
If the answer is one of the last two, the fix is a repair rather than a habit change — and knowing that early is what stops you paying for the same clearing three times.