A plumbing emergency rarely announces itself politely — it's usually a burst pipe, an overflowing toilet, or water appearing somewhere it shouldn't be, and what you do in the first few minutes matters more than people expect for limiting the damage.
The very first step, before anything else, is to shut off your home's main water supply. Every home has a main shutoff valve, typically located where the water line enters the house — near the water meter, in a basement, crawl space, or utility closet, or occasionally outside near the street. Find yours today, before an emergency happens, so you're not searching for it while water is actively spreading across your floor.
If the emergency is isolated to a single fixture — a toilet overflowing or a supply line under a sink bursting — you may be able to shut off just that fixture's local shutoff valve instead of the whole house, which is usually a small oval or round handle on the pipe leading to the fixture. This limits disruption to the rest of the home while you wait for help.
Once the water is off, address standing water quickly to limit damage: move anything absorbent or valuable off the floor, use towels or a wet/dry vacuum on standing water, and open windows or run fans if it's safe to do so, since water damage and mold risk increase the longer moisture sits. If water has reached electrical outlets or appliances, don't touch them, and shut off power to that area at the breaker panel if you can do so safely.
For a sewer backup specifically — water or waste coming up through a drain, toilet, or floor drain — stop using all plumbing fixtures in the house immediately, since running more water or flushing anything makes the backup worse. This is also the one plumbing emergency where you should avoid running a garbage disposal or any drain until a plumber has diagnosed the blockage, since a sewer line backup can indicate a problem well beyond your home's own pipes.
Once the immediate situation is controlled, call a licensed plumber and describe exactly what happened and what you've already done — which valves you shut, how long water was running, and whether it reached any electrical systems. Many plumbers offer emergency or after-hours service for exactly these situations; it's worth confirming that upfront and asking about the emergency service fee, which is typically higher than a standard scheduled visit but can be well worth it to stop damage from spreading overnight.