The first move is mechanical: close the main shutoff. Every gallon after that decision is a gallon that does not need drying, and the shutoff is the one tool every homeowner should be able to find in the dark. If water has reached outlets or appliances, cut power to those rooms at the breaker before wading in.
In Costa Mesa, the first hour includes a foundation question. If your home is raised, with a crawl space, send no one under the house, but do look through the access vent with a flashlight after the shutoff: standing water under the floor changes the repair plan. If your home is on a slab, the meter test is the evidence. Either way, photograph the failed section if it is visible in a garage or attic, because on galvanized systems that photograph starts the honest event versus pattern conversation.
The second move is evidence: photographs of the failed line, the rooms, and the water's reach, taken before anything is moved or mopped. Insurers approve documented sudden losses; undocumented ones get argued. Note the time you found it and the time the water stopped.
The third move is the repair read: a split is a symptom with a cause (freeze, corrosion, pressure, age), and the repair visit should name the cause, because a patched effect with a live cause is next season's burst in a nearby section of the same run.
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