Building a strong record of the water damage
You prove a water leak with documentation: dated photos and videos of the leak, stains, and damage; written reports to your landlord; any repair records; and receipts for damaged belongings.
Capture the water as it happens if you can — a leak during rain or an actively dripping pipe is strong evidence — along with stains, warping, sagging, and any mold that follows. Save every text and email with your landlord, since these show what the landlord knew and when. Keep records of failed or repeated repairs and note the dates.
Photos over time are especially useful, because they show whether the problem got worse while the landlord did nothing. Hold on to receipts for ruined items to show the cost. You do not need an expert to have proof; a clear, dated record usually tells the story. A free review can help you understand which evidence matters most in your situation.
This is general information about California tenant rights, not legal advice. Every situation is different.