If you are there
First steps when you are at the property
Turn off the main water valve if you know its location and can get to it safely, and stop using whatever fixture is overflowing or leaking. Keep clear of wet floors around outlets, appliances or equipment, and do not cross water to reach a breaker panel.
If the flooding has made the space unsafe, leave and call for emergency help. From somewhere safe, explain what is happening and which rooms are affected. If no tap has water at all, the city's address lookup for finding your drinking water provider can show who supplies the property.
