Basement Flooding: Find the Source Before You Pump

Basement flooding: what to do and where the water came from

Short answer

First identify the source, because it decides everything else. Clean water spreading from a wall or floor crack after rain is groundwater, and the sump pump is the first suspect. Water coming up from the floor drain, especially if it's gray or smells, is a sewer backup. Water spreading from one spot when it isn't raining is a supply leak or water heater, so close the main valve.

Safety before anything else

Electricity
Don't walk into standing water if outlets, cords or appliances are submerged or close to it. Turn off power to the basement from a dry location, or call the electric utility.
Gas appliances
If water reached a furnace or water heater, don't relight them until they're checked.
Sewage
If the water came from a drain, treat it as contaminated: boots, gloves, and keep children and pets out.

Where the water is coming from

What you see	Source	First move
Water rising from the floor drainMost common	Gray or smelly water; may happen during heavy rain or when fixtures are used.	Stop using water; main line or city sewer. See sewer backup. Sewer Backup →
Clean water at walls or cracks after rainMost common	Sump pump not running or overwhelmed; gutters dumping at the foundation.	Check the pump's power, float and discharge. Sump Pump Repair and Installation →
Water spreading from one spot, no rainPossible	Water heater, washer hose, supply pipe or softener.	Close the main valve and find the wet source. Pipe Repair →
Water after a deep freezePossible	Burst pipe, often in a rim joist or near an exterior wall.	Close the main valve; see burst pipe repair. Burst Pipe Repair →

Checking the sump pump

Is it powered?
A tripped GFCI outlet or breaker is a common reason a pump stops.
Is the float free?
The float switch can catch on the pit wall or the cord.
Is it pumping out?
A frozen or crushed discharge line, or a stuck check valve, means the pump runs without removing water.

Preventing the next one

A backwater valve on the main sewer line stops city sewer surges from coming up your floor drain. Some cities subsidize them.
A battery backup sump pump keeps working when storms knock out power, which is exactly when it's needed.
Extend downspouts several feet from the foundation and keep gutters clear.
Replace rubber washing machine hoses with braided ones, and check the water heater's age.

Call a plumber now if

Sewage or gray water is coming up from a drain.
The sump pump has failed during rain.
A pipe has burst or the water heater is leaking.