My neighbor’s toilet supply line burst at 11 PM on a Saturday. Water poured through the ceiling into the garage below for 45 minutes because nobody in the house knew where the main shut-off was. By the time the plumber arrived — $250 for an emergency call — the damage was $8,000.

I found my own shut-off valve the next morning. It took two minutes.

Where to Look

The main shut-off is usually in one of these places:

• Where the water line enters the house. Look along the front wall of your basement or crawlspace, facing the street. The pipe comes through the foundation here.
• Near the water meter. If you have a basement, the meter and shut-off are usually together.
• In a utility closet or garage. In warmer climates where pipes do not freeze, the shut-off might be on an exterior wall in the garage.
• Outside, in a ground-level box near the street. This often requires a special tool (a “water meter key,” $15 at any hardware store).

How to Test It

1. Turn the valve clockwise until it stops. Do not force it — old valves can break if you crank them too hard.
2. Turn on a faucet inside. If no water comes out, you found the right valve. Congratulations — you now know how to stop a flood.
3. Turn it back counterclockwise slowly. Open faucets inside to let air out of the pipes so they do not bang.

If Your Valve Is Stuck

Old gate valves seize up after years of not being turned. Spray penetrating oil (WD-40 works temporarily but PB Blaster is better) on the stem where it enters the valve body. Let it sit for 15 minutes. Try again gently. If it still will not budge, do not force it — call a plumber to replace it proactively. A broken valve during an emergency is worse than no valve.

After my neighbor’s flood, I wrote “WATER SHUT-OFF” on a label maker and stuck it to the wall next to the valve. Everyone in my house knows where it is and how to turn it.